The board before which Major Seaman made his plea has recently, according to the public press, reported to the Secretary of War that "The recommendation that the fresh meat ration be reduced in quantity was so opposed to all the teachings of experience, both in our country and in Cuba and Puerto Rico, that the board was unable to accept the recommendation as conclusive without further investigation. Two members of the board have served in Cuba, and the third in Puerto Rico, and their personal experience has been that as much meat has been desired and caten as in the United States, and with no deleterious effect on the health of the men. The natives of these countries are also large meat eaters when they are able to secure it, and the meat eaters are noticeably stronger and healthier looking than the poorer classes who, from necessity, are mainly vegetarians. The board also interviewed a number of officers and other persons that had been in the Philippines, and taking all sources of information together, the board is of the opinion that it would be a mistake to make any fixed reduction in the meat ration." The criticism of General Henry in the above quoted article that "The objection to the meat (of Puerto Rico) for an American is, that having no place to keep beef after being killed, it has to be put in the pot in a hot, quivering condition, and I believe this made many an American soldier ill," is in part well applied. Puerto Rican beef cannot be cooked American fashion and be other than tough and unpalatable, as it is killed about 3 o'clock in the morning and eaten by 11 or 12 o'clock the same day. I had so fully recognized this fact that for more than a year before leaving the island I refused all "bifstek" or "rostbif" as presented by the seductive native. Only Swift or Armour refrigerated beef can in Puerto Rico be prepared American fashion. Native beef, however, prepared by native cooking, is tender and palatable, devoid of this objection, and is habitually consumed with only the best results. In fact, when they ask us to replace the meat ration by vegetables, we should not forget the Spanish motto that says: "Bellotas y tostones hacen malos trabajadores.-From "Boston Medical and Surgical Journal."

RECENT ADVANCES IN THE PHYSICS OF WATER.

By GEORGE FLOWERS STRADLING, PH.D., Member of the Franklin

Institute.

From "The Journal of the Franklin Institute," October, 1901.

For a long time it has been known that water, the most common of all liquids, has physical properties which vary widely from those of most other liquids. The large quantity of heat required to melt a gram of ice, the still larger quantity required to convert a gram of water into vapor at the same temperature, and the existence of a temperature of maximum density between the melting and the freezing points, mark water as departing from the usual rules of liquid comportment. Already the list of the anomalies presented by water is long, and yet it is no uncommon occurrence to have it increased. For instance, Hauser, in the. July number of "Drude's Annalen" for 1901, shows that at 32 degrees Centigrade the viscosity of water is not changed by a pressure of 400 atmospheres, while at other temperatures it is changed.

Within the last decade, serious attempts have been made to furnish an explanation of the irregularities of water. The idea that water molecules are not H,O simply but are aggregates of this group is advanced by Raoult* as a result of his experiments upon the molecular lowering of the freezing point of solutions, and probably expressions of this view would be found long before. When, in 1891, Roentgen attacked the problem, he used this suggestion and succeeded in furnishing a qualitative explanation.†

He regards water as composed of molecules of two kinds, which he designates ice molecules and molecules of the second kind. Ordinary water is considered to be a saturated solution of ice molecules in a mass of molecules of the second kind. By the addition of heat, ice molecules are converted into those of the second kind. This is accompanied by a diminution of volume, just as when ice melts.

The Maximum Density of Water.-At 4 degrees Centigrade, water is at its maximum density. The addition of heat to a mixture of the two kinds of molecules will cause

*Raoult, Ann. chim. et phys.," Series 6, Vol. II, p. 66, 1884.

Roentgen, "Wied. Ann.," XLV, p. 91. "Uber die Constitution des flüssigen

Wassers.

(1) A diminution of volume, due to the change of ice molecules into molecules of the second kind.

(2) An increase of volume, due to the expansion of the mass of molecules of the second kind.

The observed change in the volume of water is the difference of these two defects. When heat is applied to water at o°, a relatively large number of ice molecules are changed into the other kind and the diminution of volume amounts to more than the increase. As the temperature rises, the number of ice molecules diminishes, hence fewer are changed into the second kind, and the difference between the diminution and increase grows less. At 4 degrees the two effects just balance each other, while above this temperature the increase of volume is the greater.

Roentgen gives this explanation, but does not claim to be the originator of it. Indeed, H. M. Vernon,* in the same year, gave practically the same explanation.

Pressure-Volume-Temperature Relations of Water.-As the temperature of simple liquids rises, their compressibility increases, but the compressibility of water grows less with rising temperature, and, according to the results of S. Pagliani and G. Vicentini,t reaches a minimum value at. 63 degrees Centigrade.

To explain this, Roentgen further assumes that an increase of pressure upon a mass of water kept at constant temperature results in the change of some of the ice molecules into molecules of the second kind, and that the number so changing is greater as the total number of ice molecules present is greater. This transformation causes a diminution of volume which is greater for a given increase of pressure at low temperatures, because then the water is richer in ice molecules. When, therefore, pressure is exerted upon a mass of water, the resulting change of volume is the sum of (1) A decrease of volume, due to the transforming of ice molecules into those of the second kind, and

(2) A decrease, due to the compression of the mass of molecules of the second kind.

The first of these decreases will grow smaller as the temperature rises, because with rising temperature the number of ice molecules becomes smaller. On the other hand, the second will probably become greater as the temperature goes up. At least, simple liquids. act thus. From the opposite effects produced upon the two decre*H. M. Vernon, "Phil. Mag.," Series 5, Vol. XXXI, p. 387, 1891. "On the Maximum Density of Water."

"

†S. Pagliani und G. Vicentini. "Beiblätter," VIII, p. 794. "Ueber die Compressibilität der Flüssigkeiten und insbesondere des Wassers."

ments by a rise of temperature, it is seen that their sum, which is the actually observed compressibility of water, might at some temperature have a minimum value.

The effect of pressure upon the coefficient of thermal expansion of ether, carbon disulphide and alcohol is to lower it; but for water it has been found that the mean coefficient, up to 50 degrees at least, is increased, even when the pressures amount to 2500-3000 atmospheres. To find an explanation for this anomalous effect, consider how an increase of pressure will act on both of the volume changes mentioned under the heading "The maximum density of water." The first, the diminution of volume, will be made less by an increase of pressure, because thereby the ice molecules. are converted into the other kind, so that there remains fewer of them to be changed by heat. Roentgen had no data by which to determine how pressure would affect the second, the increase of the volume of the mass of molecules of the second kind. He suggests that perhaps there would be but little effect upon the coefficient of expansion. Let us now compare the thermal expansion of a mass of water under a pressure of one atmosphere with the expansion under a higher pressure, the rise of temperature being, for example, from 5 degrees to 10 degrees Centigrade in both cases. The observed expansion under both pressures is the expansion of the aggregate of molecules of the second kind minus. the decrease in volume due to the change by heat of some ice molecules into the others. Under the higher pressure the minuend is larger because there are more molecules of the second kind in the water, and the subtrahend is less because the number of ice molecules is smaller. The remainder, the observed thermal expansion, is therefore larger for the higher than for the lower pressure.

Amagat finds that at high pressures water comports itself as an ordinary liquid. This finds its explanation in the disappearance of ice molecules. His results likewise show that the effect of pressure upon the coefficient of thermal expansion is most marked when the temperature and the pressure are low. This is understood when it is recalled that the number of ice molecules is greater at low pressure and at low temperature.

Lowering of the Temperature of Maximum Density by Pressure. The temperature of maximum density moves downward from 4 degrees Centigrade as the pressure upon the water is made greater. According to the view already advanced, at 4 degrees Centigrade, and under atmospheric pressure, the effect upon the volume of a mass of water caused by a small increase of tempera

ture is zero, because there the two opposite volume changes just balance each other. An increase of pressure lessens the number of ice molecules and consequently gives to the expansion of the mass of molecules of the second kind the preponderance. Under this increased pressure, then, the balancing of the two effects no longer occurs at 4°, but at some lower temperature, where the water is richer in ice molecules.

By pressure water can be cooled below o°C. without freezing, because the pressure prevents the formation of ice molecules.

Viscosity of Water.-Water under pressures of several hundred atmospheres is less viscous than at normal pressure, provided the temperature does not exceed 32°C. Other liquids which have been investigated grow more viscous with an increase of pressure.

The viscosity of water is generally made greater by dissolving other substances in it, and the larger the quantity of the solute the greater, in general, is the viscosity. Its viscosity would, accordingly, be expected to increase with the proportion of ice molecules. An increase of pressure by reducing their number would reduce at the same time the viscosity. An increase of temperature lowers the viscosity for the same reason, and very likely also by decreasing the viscosity of the mass of molecules of the second kind.

From the investigations of R. Cohen,* it appears that the decrease of viscosity produced by a given increment of pressure is greater the lower the temperature. For instance, at 10°C., the effect of an increase of pressure of 600 atmospheres is about six times as great as at 23°C. This is to be attributed to the greater effect at 10° of the pressure in reducing the number of ice molecules. Moreover, when the temperature is constant, he finds that a given increase of pressure produces a greater effect upon the viscosity when the pressure already exerted is small than when it is larger. As an illustration, when the temperature is 15°, an increase of pressure from 100 to 300 atmospheres produces a larger decrease in the viscosity than an increase from 300 to 600 atmospheres. The reason of this is to be found in the larger proportion of ice molecules present at low pressures, and the consequent greater reduction in their number produced by the added pressure.

These results obtained by Cohen are of especial interest, since they form the fulfilment of a prediction made by Roentgen "that the viscosity of water at high pressures will be less reduced by a given increase of pressure than at low pressures" and "that the

*R. Cohen. "Wied Ann.," XLV, p. 666, 1891. "Einfluss des Druckes auf die Viscosität von Flüssigkeiten."