1 66 POPULAR SCIENCE MONTHLY. 



view-points from which to examine the foundations of the theory. 

 There are three advances in the details of the theory with which his 

 name is generally mentioned, as follows: (1) He showed that the 

 analogy between the coefficient of diffusion for gases and the con- 

 ductivity for the propagation of heat can not be pushed to the con- 

 clusion that since the conductivity is a constant magnitude, the coeffi- 

 cient of diffusion is a constant also, for experiments with the same pair 

 of gases. Tait showed that this expectation is not justified by the 

 formula in terms of which the coefficient is deduced, showing in fact 

 that it depends at any instant not only on the temperature and on the 

 pressure of the mixture, but also on the ratio in which the two gases 

 have mixed with each other by that time. (2) He showed that in 

 applying Maxwell's law of the distribution of velocities (a law deduced 

 for a gas without total progressive or rotational motion) to the case 

 of a gas the body of which is in rotation, the interval of time within 

 which the mechanical theorems (deduced for the static conditions) 

 remain valid with sufficient exactness for a single layer (the whole body 

 of gas being considered as divided into layers between which the inter- 

 change of energy is slow) may be long enough to allow the very rapidly 

 resulting arrangment in the distribution of velocities according to 

 Maxwell's law to occur. In each of these layers, then, Maxwell's law 

 holds good for the distribution of velocities on the condition that the 

 velocity in which a particle shares by the flow or rotation of its layer 

 is to be subtracted from the value which it would have in the state of 

 rest and equilibrium of the gas as a whole. (3) The calculation of the 

 coefficient of viscosity on the assumption of Maxwell's law of distribu- 

 tion of velocities. 



In reading Tait's papers on the kinetic theory of gases it is interest- 

 ing to note the author's frank confession: "I have abstained from read- 

 ing the details of any investigation (be its author who he may) which 

 seemed to me to be unnecessarily complex. Such a course has, inevitably, 

 certain disadvantages, but its manifest advantages far outweigh them \" 



Tait's chief experimental research was that on the compressibility 

 of water, undertaken in connection with an investigation of the errors 

 of the deep-sea thermometers used on the famous voyage of the Chal- 

 lenger. It is an interesting record of a laborious investigation under- 

 taken to decide a very important practical question. 



Several earlier investigators had studied the compressibility of 

 liquids, always chiefly of water. Only the chief among them need be 

 mentioned here. Canton, a hundred and twenty-six years before, had 

 not only exhibited the compressibility of water, but had shown that 

 it decreases as the temperature is raised; and Perkins in 1826 showed 

 very clearly that in water at 10° C. the compressibility diminishes as the 

 pressure increases, quickly at first, afterwards more and more slowly; 



