144 Royal Society. 



results is 0*22977, or nearly 0*23, which we may take as the specific 

 heat of air at constant pressure determined by the above experiments." 



I had been informed that M. Regnault was working on the spe- 

 cific heat of gases, and on that account did not feel it desirable to 

 enter upon the laborious investigation which would have been re- 

 quisite in order to add a couple of decimal figures to the number I 

 had arrived at, and which was sufficient for the object I had in view, 

 viz. to show that the discrepancy between the actual and theoreti- 

 cal velocity of sound arose from the incorrectness of Delaroche and 

 Berard's determination of the specific heat of air (0*267), and not 

 from any notable error in my number for the mechanical equivalent 

 of the thermal unit. Having succeeded in doing this, I calculated 

 the Tables 3 and 4 of my paper, using 0- 238944 for the specific heat 

 of air under constant pressure. I feel much gratified that the result 

 arrived at by so eminent an experimentalist as M. Regnault confirms 

 the accuracy in the main of the number I adopted. 



I have only to add that Professor Thomson and myself, in pursu- 

 ing our research on the thermal effects of rushing elastic fluids, are 

 following up the views on the relation between mechanical and 

 thermal phenomena originated by ourselves ; and we shall feel most 

 happy if M. Regnault's results, in the important line of investiga- 

 tion he has adopted, will facilitate our labour. 



I have the honour to remain, dear Sir, 



Yours most truly, 



Colonel Sabine, SfC. SfC. %c. J. P. Joule. 



2. " Experimental Researches on Vegetation." By M. Georges 

 Ville. 



After stating that it has often been asked if air, and especially 

 azote, contributes to the nutrition of plants ; and, as regards the 

 latter, that this question has always been answered negatively, the 

 author remarks it is however known that plants do not draw all 

 their azote from the soil, the crops produced every year in manured 

 land giving a greater proportion of azote than is contained in the 

 soil itself. The question which he has proposed to himself for so- 

 lution is, whence then comes the excess of azote which the crops 

 contain, and in a more general manner, the azote of plants, which 

 the soil has not furnished ? He divides his inquiry into' the three 

 following parts : — 



First. Inquiry into and determination of the proportion of the 

 ammonia contained in the air of the atmosphere. 



Second. Is the azote of the air absorbed by plants ? 



Third. Influence on vegetation of ammonia added to the air. 



1. The author remarks that since the observation of M. Theodore 

 de Saussure, that the air is mixed with ammoniacal vapours, three 

 attempts have been made to determine the proportion of ammonia 

 in the air : a million of kilogrammes of the air, according to M. 

 Grayer, contain 0*333 kil. N'~H 3 ; according to Mr. Kemp 3*880 

 kil. ; according to M. Fre'senius, of the air of the day, 0*098 kil., and 

 of night air, 0*169 kil. He states that he has shown the cause of 

 these discrepancies, and proved that the quantity of ammonia con- 



