196 Dr. Apjohn on the Specific Gravity of Gases. [Sepv. 
at a given temperature F’, the specific gravities are as the ten- 
7.4484 F 
= o / ; -- = 
sions, we shall have [3-4 : 7’ any other tension :: s the spe 
cific gravity corresponding to the former tension : s’ = s 
+! A484 F 
7° 448+" 
it being ascertained (according to Mr. H.) “ by the concurrent 
experiments of the French and English philosophers, that, with 
the exception of their not being able to sustain more than a cer- 
tain pressure according to the temperature, vapours are perfect 
gases, and follow precisely the same laws of expansion and con- 
traction,” he easily infers that the specific gravity of steam is not 
as its elasticity. With deference, however, to Mr. H. he has, I 
must say, though no doubt unintentionally, misrepresented the 
the specific gravity corresponding to the latter. Now 
philosophers. It is true they have shown, that vapours apart. 
from their respective fluids, “‘ obey the same laws of expansion 
and contraction with the otber gases.” But the case is far differ- 
ent with vapours in contact with their fluids. The effects of an 
increment of temperature upon gases, or vapours apart from, and 
those same vapours in juxtaposition with their fluids, are strik- 
ingly distinct. The volume being given, the elasticity of the 
ras Is augmented, but not its density. On the other hand, not 
only the elasticity, but also the density of a vapour in contact 
with its fluid is increased. Let us return now to the above-men- 
tioned proportions of Mr.H. The first evidently does not apply 
to vapours over fluids, for the volume being given, it supposes 
the density also given, which in the case of vapours so situate is 
not the fact. The second is also without meaning here, or is at 
best but a trifling proposition ; for what does it state? Why, that 
if the temperature be given, the density is as the tension. But 
the tension ofa vapour in contact with its fluid is always the same 
at the same temperature, and, therefore, so must the density. 
Now to say that a varies as 6, when neither a or 6 vary at all, is 
certainly little short of being absurd. The original proposition, 
therefore, namely, that the density of steam is as its tension, has 
not been shaken by Mr. Herapath, for the result which he arrives 
at, and from which he deduces zs falsehood, does not apply to 
vapours in contact with their fluids. It still, however, may be 
doubted whether the density and tension of steam are so simply 
related, for Mr. H.’s failure to prove the negative does not esta- 
blish the affirmative of the proposition. A few accurate experi- 
ments, by determining the specific gravity of steam at different 
temperatures, would enable us (as we are already possessed of 
tables of elasticities) to bring this law to the test of experience. 
Gay-Lussac indeed having already determined its specific gravity 
at 212, another determination would afford at least a single 
comparison. It will be observed that I do not any where assert 
that the density of steam is precisely as its elasticity. Mr, H. 
