350 
DR. DALTON ON THE CONSTITUTION OF THE ATMOSPHERE. 
Common air and hydrogen in which the oxygen is only -J T th, or seven per cent., ex- 
plode imperfectly, leaving both oxygen and hydrogen. 
Common air and hydrogen in which the oxygen is from ^th to 6 4 6 th, or from eight 
to fourteen or fifteen per cent., fire leaving hydrogen and azote only. 
Common air and hydrogen in which the hydrogen is d^th to yth, or from fourteen 
to thirty per cent., fire and leave oxygen and azote only. 
Common air and hydrogen in which the hydrogen is Ath to -rh-th, or from eight to 
twelve per cent., fire imperfectly, and leave oxygen, hydrogen, and azote. 
Common air and hydrogen in which the hydrogen is -yVth or less than seven per 
cent., do not explode. 
It should be observed that when one of the gases is so far deficient as not to allow 
of an explosion by a single spark, the effect may be obtained by a current of sparks 
for a longer or shorter period, accompanied by the requisite diminution of volume. 
In such instances where the effect is produced only by a current of sparks it may be 
proper here to suggest the reason. When mixtures explode perfectly but feebly, we 
see the flame, lighted by the spark, to run down the eudiometer till it reaches the 
water ; when they explode still more feebly, the flame runs perhaps half way down 
the tube and is extinguished before it reaches the water. There scarcely can be a 
doubt that the extinction must be occasioned by the cooling effect of the eudiometer 
and of the intermixture of the mass of air which has to be heated by the feeble flame. 
Another spark in its passage will re-alight the flame, to suffer a quicker extinction, 
and so on till at length the combustion is complete. This reason will also explain 
the excessively slow combustion of azote by the electric spark, as ascertained by 
Mr. Cavendish, and as I have found by repeated experience. Query, might not this 
experiment succeed better by heating the eudiometer ? 
From what we have stated it must be obvious that in order to secure the complete 
abstraction of either oxygen or hydrogen from mixtures by Volta’s eudiometer, we 
should avoid too near an approach to the limitations we have pointed out ; or if that 
cannot be, we should carefully examine the residue for both gases. The best test for 
very small portions of oxygen is undoubtedly nitrous gas ; for somewhat larger por- 
tions of oxygen or hydrogen, additions of those gases might be made so as to bring 
the mixtures into proportions capable of being exploded. 
Second Method, by Nitrous Gas. 
The nitrous gas eudiometer is of singular utility on many occasions. No other 
can exceed it in accuracy when mixtures contain very little, as one or two per cent, 
of oxygen ; or on the other hand when nearly the whole of the gas is oxygen. But 
when the mixture of gases contains from twenty to eighty per cent, of oxygen, as in 
the case of common air, it is not the best when great exactness is required. The 
reason is well known ; when oxygen and nitrous gas combine, the combination is not 
like that of oxygen and hydrogen, in uniform proportion. We may take one third of 
