Prof. TyndalPs Notes on Scientific History. 45 



equivalents of the chemical forces actually operating during 

 those times" *. 



Comparing the foregoing remarks with what Mayer had 

 written seven years earlier, the reader will draw his own conclu- 

 sions as to their comparative completeness. In his writings 

 upon vital dynamics Prof. Thomson never once mentions the 

 name of Mayer ; he is not, I presume, to be blamed for this 

 omission; for when he wrote in 1852 he knew nothing about 

 Mayer's most important labours. I state this because the 

 opposite supposition is too unpleasant to be entertained. But 

 in 1862 I gave him the titles of Mayer's memoirs, and requested 

 him and others to refer to them, and correct me if I had erro- 

 neously estimated the merits of their author. To my great 

 regret, Prof. Thomson, without giving himself the trouble of 

 consulting the documents to which I had referred him, sanctions 

 the publication of the statement, that, as far back as 1851, he had 

 given to Mayer " the full credit which his scientific claims can 

 possibly be admitted to deserve " f. 



52. In the paper from which I have just quoted, Prof. "Win. 

 Thomson also refers to the deoxidation of carbon and hydrogen 

 from carbonic acid and water, effected by the action of solar 

 light upon the green leaves of plants, as a mechanical effect of 

 radiant heat. This action, he says, was pointed out by Helm- 

 holtz in 1847. 



53. The words of Helmholtz are as follows (Erhaltung der 

 Kraft, p. 69 J) : — " There remains to us of known natural pro- 

 cesses those of organic beings. In plants the processes are 

 principally chemical, and besides this, in some of them, at least, 

 a slight generation of heat takes place. Of foremost importance 

 is the fact, that in them a great quantity of chemical forces is 

 deposited, the equivalent of which we obtain as heat on the com- 

 bustion of the plant. The only vis viva which, according to our 

 present knowledge, is absorbed during the growth of the plant, 

 consists of the chemical rays of the sun. Results are, however, 

 still wanting to enable us to make a sure and strict comparison 

 of the forces which here disappear and appear. For animals we 

 have, however, some grounds of comparison. They take in the 

 complicated oxidizable compounds which are produced by plants 

 and oxygen, and return them for the most part burnt as carbonic 

 acid and water ; partly, however, they are excreted, reduced to sim- 



* On first reading this passage I thought it might be an abstract of a 

 fuller statement, but I have been unable to find anything more complete. 



t Phil. Mag. vol. xxv. p. 264. 



X This excellent essay was translated by myself many years ago, and 

 published in the last volume of the Scientific Memoirs. (Taylor and 

 Francis, Red Lion Court, Fleet Street.) 



