up the anhydrous salt until -0°45 was reached; then the cryohydrate was formed. The amount of salt was determined by evaporation over sulphuric acid in vacuo. 3.8835 gave 0.2892 or 7.45 This percentage exhibits the water-worth of 174. The same salt as a cryogen reaches the temperature -0°6. § 180. Acetate of Zinc.-A boiling saturated solution was allowed to cool to the atmospheric temperature (+15°). The residual liquid was drained from the separated crystals and introduced, together with a few crystals, into a stoppered bottle. After keeping in ice for nine hours with frequent agitation, all sign of further crystallization ceased. The zinc was estimated by precipitation with carbonate of sodium. It was thus found that the strength at 0° was 23 per cent. This solution, when artificially cooled, yields fern-like crystals at -59; these gradually became opaque, the opacity forming in a frond-like manner; the whole became dry at the above temperature. An analysis of the residual liquid, after two crops of cryohydrate, showed that the cryohydrate had sensibly the same composition as the solution saturated at 0° C.; for 5-9980 gave 0-6103 or 23 per cent. anhydrous acetate. § 181. Hyposulphite of Soda.-The following are the temperatures at which solidification begins in solutions of varying strengths of this salt: A 30-per-cent. solution does not always give up a solid at -90.5. Sometimes the temperature sinks to -12°·4; then the true cryohydrate is formed, and the temperature rises to —11° and remains constant till all is solid. The solubilities at 0° and 20° are those given by Kremers. These and the determination for -11°, when plotted in the usual way, are found to lie on the same straight line. H. Schiff found at 19°5 a saturated solution to contain 63.5 per cent. five-hydrated, or 45-8 per per cent. anhydrous salt. As a cryogen, the temperature -10° was reached. § 182. Citric Acid. This body presented many difficulties; but as these difficulties occur again with most organic acids of high molecular weight, a special study was made of it. It is peculiarly liable in aqueous solution to supersaturation of the most persistent kind, especially when the solution is at a low temperature. At temperatures and under conditions. which are capable of evolving the cryohydrate, the solution assumes sometimes an almost colloidal form, and shows no signs of eliminating solids unless other means besides mere lowering of temperature are employed. From solutions ranging from 10 to 40 per cent. of anhydrous acid, ice is liberated; and this continues to 42.26 per cent., from which solution a cryohydrate separates at -90.2. The following is a somewhat detailed account of the behaviour of solutions containing a greater percentage of acid than 40. Two grams of a solution gave 0-8525 of citric acid, or 42.28 per cent. This gives a solid at -9°2, which at first floats on the residual liquid. The solid consists of massive white agglomerated crystals. The crystals are hexagonal, but present rhomboidal elements, causing the edge of each crystal to be deeply and regularly serrate. When they melt, the rhomboidal crystals are themselves resolved into long slender prisms. A large quantity of such a solution retained its composition when nine tenths of it had been removed by solidification, nevertheless, if such a solution be kept perfectly still for many hours at -9°, a few ice spicula may be formed. Other solutions, containing respectively 45, 45.9, 50, 50·7, and 51.5 per cent. of the anhydrous acid, were examined. It is only this latter which yields, on cooling, distinct quantities of the original salt: this it does at 60, but only if a particle of the original salt be introduced and by diligent stirring. When undisturbed, this 51.5 solution may be cooled to -19°5 without any solidification. So prone is this acid to exhibit supersaturation, that solutions both weaker and stronger than the 42.62 may be enriched on partial solidification. Thus a 50-per-cent. solution, though already stronger than the cryohydrate, may become still stronger by the separation of ice at -17°. There is therefore a large region of double supersa turation; the ice-curve crosses the acid or suberyohydratecurve, both continuing their courses for an exceptionally long distance. For the solubility at 0° C., a solution saturated above 0° C. was kept at 0° surrounded by ice and placed in an ice-safe for three days. Solution. Anhydrous - 2.0962 gave 1·0755 or 51.30 2.1288 Crystallized citric acid when added to water has a considerable cooling effect. Thus 110.9 grams of crystallized acid at 20°.5 C., added to 16°.5 lowered the temperature to 20.5; 89 while 51-5 grams of anhydrous acid cooled to 0° C. and added to water cooled to 48.5 0° The chief results may be summarized in the following Table: As a cryogen, the lowest temperature attainable is -90.3; and this confirms the composition of the cryohydrate which had been deduced synthetically. Neither cooling the acid to 0° C. nor cooling the two separately to -9° C. had any effect upon the temperature; but, of course, the more nearly the initial temperature is to the final one the less is the quantity of liquid formed. Miscellaneous Notes. § 183. The following notes of salts which have not yet been fully examined may be useful. Cyanide of Potassium, as a cryogen, gives a temperature of -21°1. The cryohydrate forms at -330, with a carbonicacid and-ether cryogen. Compare § 170. Oxalate of Sodium forms a cryohydrate at -10.7 C. Employed as cryogens, the following temperatures were obtained from the corresponding salts: Those of these bodies which evolve heat on mixture with water would, when cooled, depress the temperature more. Thus the chloride of manganese scarcely showed signs of a cryohydrate at -40° C. Formate of Sodium, as a cryogen, gives -14°3. A concentrated solution becomes semisolid at -14°, but does not become opaque or completely solid in a salt-ice cryogen (-22°). Tannic Acid, as a cryogen, gives -1°.5. Sulphurous Acid gives a cryohydrate at -10.5. Boracic Acid, as a cryogen, gives -0°.8. The cryohydrate forms at 0°·7. Arsenious Acid.-The cryogen stands at -0°-3; the cryohydrate formed at -0°5. Two samples of the melted and liquid cryohydrate were sealed hermetically. After two or three days it was found that a considerable quantity of a fine white powder had exhibited itself. IN [To be continued.] V. On the Physical Action of the Microphone. N the paper read on the 9th of May before the Royal Society, I gave a general outline of the discoveries I had made, the materials used, and the forms of microphone employed in demonstrating important points. I have made a great number of microphones, each for some special purpose, varying in form, mechanical arrangement, and materials. It . Communicated by the Physical Society, having been read June 8, 1878. would require too much time to describe even a few of them and as I am anxious in this paper to confine myself to general considerations, I will take it for granted that some of the forms of instrument and the results produced are already known. ; The problem which the microphone solves is this-To introduce into an electrical circuit an electrical resistance, which resistance shall vary in exact accord with sonorous vibrations so as to produce an undulatory current of electricity from a constant source, whose wave-length, height, and form shall be an exact representation of the sonorous waves. In the microphone we have an electric conducting material susceptible of being influenced by sonorous vibrations; and thus we have the first step of the problem. The second step is one of the highest importance: it is essential that the electrical current flowing be thrown into waves of determinate form by the sole action of the sonorous vibrìtions. I resolved this by the discovery that when an electric conducting matter in a divided state, either in the form of powder, filings, or surfaces, is put under a certain slight pressure, far less than that which would produce cohesion and more than would allow it to be separated by sonorous vibrations, the following state of things occurs. The molecules at these surfaces being in a comparatively free state, although electrically joined, do of themselves so arrange their form, their number in contact, or their pressure (by increased size or orbit of revolution) that the increase and decrease of electrical resistance of the circuit is altered in a very remarkable manner, so much so as to be almost fabulous. The problem being solved, it is only necessary to observe certain general considerations to produce an endless variety of microphones, each having a special range of resistance. The tramp of a fly or the cry of an insect requires little range but great sensitiveness; and two surfaces, therefore, of chosen materials under a very slight pressure, such as the mere weight of a small superposed conductor, suffice; but it would be unsuitable for a man's voice, as the vibrations would be too powerful, and would, in fact, go so far beyond the legitimate range that interruptions of contact amounting to the wellknown "make and break " would be produced. The A man's voice requires four surfaces of pine charcoal, as is described in my paper to the Royal Society, six of willow charcoal, eight of boxwood, and ten of gas-carbon. effects, however, are far superior with the four of pine than with either the ten of gas-carbon or any other material as yet used. It should be noted that pine wood is the best resonant material we possess; and it preserves its structure and quality |