210 REPORTS ON THE STATE OP SCIENCE. 



which need not be considered in detail here it is possible to calculate 

 the number of collisions with its neighbours which the average molecule 

 undergoes per second. This calculation can be approached in various 

 ways, based on different kinds of data, but they all lead to the same 

 result, at any rate as regards order of magnitude — namely, that a mole- 

 cule of air at normal temperature and pressure collides on the average 

 3 x 10 9 times per second with other molecules. At every collision the 

 energy distribution in the colliding molecules is modified, both 

 as regards the manner in which it is shared between the two and the 

 relative proportions due to vibration and translation in either. It is 

 argued that after every molecule has suffered a few thousand collisions, 

 which will happen in a millionth of a second, the gas must have reached 

 a steady average state. This argument would, however, be upset if 

 the interchange of energy as between vibration and translation at each 

 collision were sufficiently small. It is only necessary to suppose that 

 a vibrating molecule loses less than one thousand millionth part of its 

 vibratory energy at each collision, to raise the time of relaxation to 

 something of the order of a second. Any objection to this supposition 

 must be founded on some hypothesis, which cannot be other than 

 entirely speculative, as to the mechanism of a collision. The kinetic 

 theory, therefore, can give no information about the absolute value of 

 the time of relaxation, though it provides valuable suggestions as to 

 the way in which that time is affected by the temperature and density 

 of the gas. 



There is plenty of physical evidence, however, that in ordinary 

 circumstances the time of relaxation is excessively short. The 

 phenomenon of the propagation of sound shows that compressions and 

 rarefactions of atmospheric air may take place many thousands of times 

 in a second without the gas departing appreciably at any instant from 

 the state of equilibrium. The experiments of Tyndall, in which an 

 intermittent beam of radiant energy directed through the gas caused 

 variations of pressure sufficiently rapid to give sounds, show that the 

 transformation of vibrational into pressure energy under the conditions 

 of his experiments is a process far more rapid than any with which 

 we are accustomed to deal in the gas-engine or in the study of gaseous 

 explosions. The departure from equilibrium which follows combus- 

 tion is, however, of a special kind, and it may be that the gas is slower 

 in recovering from it than when the disturbance is that produced by 

 the propagation of sound at ordinary temperatures. 



Transparency . 



The radiation from hot gas is complicated by the fact that the gas 

 is to a considerable extent transparent to its own radiation. The 

 radiation emitted, therefore, depends upon the thickness of the layer 

 of gas, instead of being purely a surface phenomenon, as in the case 

 of a solid body. This property, besides being of great physical interest, 

 is important from the point of view of the Committee because upon it 

 depends, or may depend, the relative magnitude of radiation losses in 

 engines or explosion vessels of different sizes. 



