456 



Mr. W. Crookes. 



[Feb. 17, 



760 millims. Carbonic anhydride, however, is different ; the propor- 

 tion between it and air holds good between 760 and 650 millims. Then 

 it gets lower and lower as the pressure sinks, until 50 or 55 millims. 

 is reached, when the proportion between it and air becomes constant. 



Hydrogen, however, is entirely different to the other gases ; its log 

 decrement remains the same to a very high exhaustion, and, that of 

 other gases sinking, it is evident that the proportion between this gas 

 and any other is different for each pressure. 



It must not be forgotten that the pressure of 760 millims. is not one 

 of the constants of Nature, but is a purely arbitrary one, selected for 

 our own convenience when working near the level of the sea. In the 

 diagrams accompanying the paper the author has started from, this 

 pressure of 760 millims., and has given the log decrement curves 

 which approximately represent the viscosities through a wide range of 

 exhaustion. But the curves might also be continued, working down- 

 wards instead of upwards. From the shape and direction in which 

 they cut the 760 line it is reasonable to infer their further progress 

 downwards, and we may assume that an easily liquefiable gas will 

 show a more rapid increase in viscosity than one which is difficult to 

 liquefy by pressure. For instance, hydrogen, the least condensible of 

 all gases, shows no tendency to increase in log decrement by pressure. 

 Oxygen and nitrogen, which are only a little less difficult to condense 

 than hydrogen, show a slight increase in log decrement. Carbonic anhy- 

 dride, which liquefies at a pressure of 56 atmospheres at 15° C, increases 

 so rapidly in log decrement that at this pressure it would have a log 

 decrement of about 1*3, representing an amount of resistance to 

 motion that it is difficult to conceive anything of the nature of gas 

 being capable of exerting. 



Kerosoline vapour is rendered liquid by pressure much more readily 

 than carbonic anhydride. Its curve shows a great increase in density 

 for a very slight access of pressure. 



Again, aqueous vapour is condensible to the liquid form with the 

 greatest readiness ; and the almost horizontal direction of the curve 

 representing aqueous vapour mixed with air carries out the hypothesis. 



It follows, then, that Maxwell's law holds good for perfect gases. 

 The disturbing influence spoken of in the commencement of this 

 paper as occasioning a variation from Maxwell's law, is the tendency 

 to liquefaction, which prevents us from speaking of any gas as 

 " perfect," and which hinders it from obeying Boyle and Mariotte's 

 law. The nearer a gas obeys this law the more closely does it conform 

 to Maxwell's law. 



Maxwell's law was discovered as the consequence of a mathematical 

 theory. It presupposes the existence of gas in a "perfect" state — a 

 state practically unknown to physicists, although hydrogen gas very 

 nearly approaches that state. An ordinary gas may be said to be 



