DR. DALTON ON THE CONSTITUTION OF THE ATMOSPHERE. 
349 
exactly; but the reason is easily discovered, and it proves the accuracy of the obser- 
vations. 
Hydrogen gas is rarely obtained quite pure : it frequently holds two or three per 
cent, of common air, detached from the water through which it bubbles and by other 
means; this air increases as more water enters the hydrogen bottle, till sometimes 
it amounts to ten per cent, at the last, as every one knows who has had a due share 
of experience. Now as Mr. Cavendish does not mention the purity of his hydrogen, 
we must try it by the means now generally known, as the reported results will guide 
us in the investigation. 
On looking at the column headed “ diminution on firing” it is easy to see there is 
a discrepancy in the first three experiments in that column ; if the hydrogen used 
contained any oxygen the diminution on firing ought to have continually decreased, 
whereas if was greater in the third than in the second experiment. This it must be 
allowed is a proof of inaccuracy in one or both of the experiments ; but it is no 
greater error than usually occurs if we trust to a single experiment with any gaseous 
mixture. The average of two or three experiments on mixtures of the same propor- 
tions should be taken. The fourth experiment clearly shows that the hydrogen con- 
tained oxygen as well as azote; for a diminution of 6T2 would denote the union of 
204 oxygen with 40'8 hydrogen ; hence there must have been T5 common air in the 
hydrogen. I have formed the column “ amendment” by assuming the hydrogen in 
all the experiments to contain 4\ per cent, common air. If we combine the results 
of the third and fourth experiments, either by assuming Mr. Cavendish’s diminution 
or that of the amendment, we shall obtain a very good approximation to the quantity 
of oxygen in atmospheric air, the former experiment giving too great diminution by 
reason of the excess of hydrogen and that containing some oxygen, and the latter 
giving too little diminution for want of the requisite quantity of hydrogen ; the former 
will give 20'98 per cent, oxygen, and the latter 20’92 per cent, oxygen in atmospheric 
air. If any doubt should remain as to Mr. Cavendish’s hydrogen containing oxygen, 
it is removed by the consideration that his first experiment would indicate 22‘9 oxygen 
per cent, in air, which cannot be allowed ; and his last experiment that 8 - 8 oxygen 
must have combined with 20’6 hydrogen instead of 17 ' 6 , which is equally inadmissible. 
Since the period 1/84 it has been found by various chemists that in mixtures of 
oxygen and hydrogen, as well as in other similar ones, the electric spark does not 
always cause an explosion, and when it does a complete combination does not always 
take place, but that in the residue sometimes portions of both the ingredients may 
be found. The limitations and restrictions are now pretty generally known ; and 
with regard to the mixtures of common air and hydrogen, I published a letter in the 
10th volume of the Annals of Philosophy, (New Series) page 304, in which I showed 
the limitations found by my own experience to be as under : 
Common air and hydrogen in which the oxygen is only -Artlb or fi’om six to seven 
per cent, of the whole mixture, do not explode. 
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