•94 ACQUAINTANCE WITH MECHANICAL 



previous, expressing his ideas in suggestive but somewhat 

 vague language. But this procedure, which was the only 

 course where the ideas were entirely new, as now applied to 

 old ideas, for which definite expressions have been adopted, 

 not only conveys a wrong impression as to his meaning, but 

 lays him open to the criticism of ignorance of the first prin- 

 ciples of mechanical philosophy. 



The expression (used in the paragraph already quoted) 

 " that the velocity " of the whirling atmospheres, by which 

 he assumes the respective atoms of the gas surrounded, 

 " determines what we call the temperature," suggests, but 

 does not fully justify, such criticism. But he has given 

 a previous description of his hypothesis of whirling atmos- 

 phere in an appendix added on February 20th, 1844, to his 

 paper of January 4th, 1843, on "The Heat Developed by the 

 Electrolysis of Water." And here he not only makes a 

 similar statement, but goes on to say that the momentum of 

 the atmospheres constitutes "caloric," and later on says "that 

 the momentum of the revolving atmospheres of electricity 

 in a pound of water at freezing is equal to a mechanical 

 force able to raise a weight of about 400,0001b. to a height 

 of one foot." If he used the term momentum in its accepted 

 mechanical sense as implying the product of mass mutiplied 

 by velocity, these passages certainly show that he was, at that 

 time, in error as to the principles of mechanics. That he 

 did use momentum in this sense is implied in his definition 

 of temperature quoted above. It is confirmed also by the 

 fact that a year later, at the conclusion of another paper, 

 he goes somewhat out of his way to recur to this subject, 

 pointing out definitely that the u vis viva " of the atmos- 

 phere will be proportional to the square of the velocity, and 

 then follows with " We see then what an enormous quantity 



