considered being normal sections), then z will be proportional to m − (1 − n) t, x and the coefficient of interdiffusion K is defined by the equation 2 = K m + 12 - I (2) The numerical quantity m+n-1 may be regarded as measuring the difference of states of the two sections A and B. If y now denote the thickness of a stratum in the confy dition of B which would be reduced to the state existing at A by the abstraction of a thickness z of the second substance, and the addition of the same thickness of the first, we have (1-n)y+z as the expression for the quantity of the first substance in the stratum after the operation. This is to be equal to my. Hence we have and substituting for z its value in (2) we have finally (3) (4) which is of the same form as equation (1), y now denoting the thickness of a stratum of the mixture as it exists at B, which would be reduced to the state existing at A by the addition to it of the quantity of one substance which diffuses from A to B in the time t, and the removal from it of the quantity of the other substance which diffuses from B to A in the same time. 126. The following values of K in terms of the centimetre and second are given in Professor Clerk Maxwell's University af 9MICHIGAN 'Theory of Heat,' 4th edition, p. 332, on the authority of Professor Loschmidt of Vienna. Coefficients of Interdiffusion of Gases. Carbonic Acid and Air, *1423 k Sulphurous Acid and Hydrogen, 127. These may be compared with the value of for с air, which, according to Professor J. Stefan of Vienna, is *256. The value of k for air, according to the same authority, is 5.58 x 10, and is independent of the pressure. Professor Maxwell, by a different method, calculates its value at 5'4 x 10-5. Results of Experiments on Conductivity of Solids. 128. Principal Forbes' results for the conductivity of iron (Stewart on Heat, p. 261, second edition) are expressed in terms of the foot and minute, the cubic foot of water being the unit of thermal capacity. Hence the value of Forbes' unit of conductivity, when referred to (30.48)2 or 15:48; and his results must be C.G.S., is " 60 multiplied by 15'48 to reduce them to the C.G.S. scale. His observations were made on two square bars; the side of the one being 1 inch, and of the other an inch. The results when reduced to C.G.S. units are as 129. Neumann's results ('Ann. de. Chim.' vol. lxvi., p. 185) must be multiplied by 000848 to reduce them to our scale. They then become as follows::- In the same paper he gives for the following substances k the values of or ; that is, the quotient of conductivity k sd с by the thermal capacity of unit volume. These require the same reducing factor as the values of k, and when reduced to our scale are as follows :- 130. Sir W. Thomson's results, deduced from observations of underground thermometers at three stations at Edinburgh (Trans. R. S. E.,' 1860, p. 426), are given in terms of the foot and second, the thermal capacity of a cubic foot of water being unity, and must be multiplied by (30.48)2 or 929 to reduce them to our scale. The following are the reduced results : k My own result for the value of from the Greenwich underground thermometers ('Greenwich Observations,' 1860) is in terms of the French foot and the year. As a French foot is 32'5 centims., and a year is 31557000 seconds, the reducing factor is (325)231557000; that is, 3*347 × 10−5. The result is Gravel of Greenwich Observatory Hill, k *01249 Professors Ayrton and Perry ('Phil. Mag.,' April, 1878) determined the conductivity of a Japanese building stone (porphyritic trachyte) to be 0059. 131. Ångström, in 'Pogg. Ann.,' vols. cxiv. (1861) and cxviii. (1863), employs as units the centimetre and the minute; hence his results must be divided by 60. These results, as given at p. 429 of his second paper, will then stand as follows: k Value of 84476 for copper; 88620 for iron, and thus deduces the following values of k :- 132. In Professor George Forbes's paper on conductivity ('Proc. R. S. E.,' February, 1873) the units are the centim. and the minute; hence his results must be divided by 60. Thus reduced, they are: |