1826.] Mr. Rainy* s Reply to Dr. Thomson. 



parts water to 400 of the strongest liquid acid, or to 400 anhy- 

 drous acid ; but even on the latter supposition, if my experiment 

 is correct, its boihng point must have been considerably below 

 224°; and as the experiment was twice repeated without any 

 material variation, I do not see any reason to doubt its accuracy. 

 If Dr. Thomson took the boiling point of his acid in a glass 

 vessel, it would account in some degree, though by no means 

 entirely, for the difference in our results. I think it unneces- 

 sary, however, to make any further remarks on the experiment 

 detailed in the Principles, as Dr. Thomson himself seems now 

 not entirely satisfied with it ; and as he states that out of ten 

 similar experiments, this was the only one that appeared to yield 

 satisfactory results, I am somewhat surprised that he should 

 ever have placed much confidence in it, and especially that he 

 should have adopted it as a basis on which to raise such an 

 important superstructure. 



In the new experiment which Dr. Thomson details in his last 

 paper, he endeavours to deprive the hydrogen entirely of mois- 

 ture, by passing it through 37 inches of tube filled with chloride 

 of calcium, which he had found was sufficient to render the 

 hydrogen gas as dry as it could be made by this method. There 

 are several objections to our admitting this experiment as suffi- 

 ciently delicate to enable us to decide on the truth of Dr. Prout's 

 hypothesis. 



1. The great weight of the flask, and tubes and their contents, 

 compared with that of the hydrogen to be weighed. The appa- 

 ratus with which the beam was loaded weighed more than 

 a thousand times as much as the whole of the hydrogen ; it is 

 not to be expected, therefore, that the absolute weight of the 

 hydrogen could be ascertained with minute precision. This has 

 been distinctly adverted to by Dr. Thomson himself. 



2. The manner in which Dr. Thomson removes the hydrogen 

 in the tubes, and replaces it with common air, is liable to 

 objection. When Dr. Thomson inhales air saturated with mois- 

 ture at about 60° through 37 inches of tube filled with the chlo- 

 ride of calcium, some of the moisture must be retained, and 

 must add to the weight of the tube, and consequently lead the 

 experimenter to underrate the weight of the hydrogen which has 

 escaped. In order to obviate this source of error, the air 

 inhaled should have been previously dried by chloride of cal- 

 cium. 



3. It results from this experiment, even supposing it quite 

 correct, that the specific gravity of hydrogen is to that of oxygen 

 as 1*0077 to 16, or in other terms as 1 to 15-87, This certainly 

 approxinates pretty nearly to the ratio of 1 to 16 ; but still the 

 difi'erence is by no means immaterial. The deviation from an 

 integer is 0'12, or about one-eighth, which seems small, but we 

 must remejaiber that the utmost possible deviation is 0*5, for were 



