1830.] 
Remarks on Daniel’s Hygrometer . 
19 
ether is a mixture of alcohol and ether, and being, in consequence of the heat of the 
climate and the greater volatility of the ether, subject to deteriorate, becomes, in a 
short time, perfectly powerless for all purposes of hygrometry. 3rd. That a simple 
and infallible test is at hand, by which the purchaser of ether may always ascertain 
its value (I mean as a hygrotnetrical agent). It is, to take a thermometer with a 
small bulb, coat it with cambric muslin, and then pouring ether on it, observe the 
maximum depression produced. If this does not amount to between 50° and 60° in 
the dampest weather, the ether is not so good as it might be. If it does not amount 
to 49° he had, probably, better not meddle with it. In dry weather the depression 
will be something greater. 
Though it be evident, from what precedes, that I had so far underrated the value 
of Daniel’s Hvgiometer in India, I am still of opinion, that it is an instrument fit- 
ter for the laboratory or the closet, than for the purposes of the traveller or the every 
day observer. As a standard instrument, it is both elegant and effective ; but for ob- 
servations to be frequently repeated, it is (to say nothing of the expense of the ether, 
or even of the difficulty of procuring it of a sufficiently good quality in India,) far 
from convenient. In this respect, the moist bulb thermometer is certainly prefera- 
ble ; but let the indications of the latter he checked by frequent comparisons with the 
former. That it is not easy to procure ether of the proper quality I know, from the 
testimony of several friends who have been equally disappointed with myself in at- 
tempting to make use of this instrument. In your third number, p. 82, another of 
your correspondents makes the following remark : “ In atmospheres, nearly deprived 
of moisture, Daniel’s method becomes nearly useless, from the great degree of cold 
required to produce deposition.” This remark would not, perhaps, have been made, 
had such ether, as that with which we experimented, been available. 
Daniel's instrument, certainly does require the most powerful ether, from the in- 
direct manner in which it is applied to produce the effect. It is applied not to the 
part of the instument required to be cooled, but to a distant part connected with it ; 
much of the power of refrigeration is by this means lost. The circumstance has, in 
fact, been made an objection to this form of the instrument. In your 10th number, 
you gave insertion to a short notice l sent you of some improvements proposed on 
this view of the subject. Of these, I am inclined to prefer Mr. Gumming’ 8, which there 
is little doubt would afford results, even with the worst ether, perhaps even with pure 
alcohol; a still simpler arrangement would be the following : — Suppose an exceedingly 
thin glass capsule, of such depth and surface as would ensure the ether poured into 
it speedily cooling many degrees below the temperature of the air ; a very delicate 
thermometer suspended in this would indicate the precise temperature at which mois- 
ture should become visible on the sides of the glass capsule. In this way no more 
ether would be used than what was absolutely necessary, the apparatus would be ex- 
ceedingly portable, and it is probable it would be an effective instrument even with 
very bad ether. A hemispherical cup, similar to the half of the dew-ball of Daniel’s 
instrument, with a thermometer, having a very small globular bulb, would, I should 
think, answer completely ; saline mixtures would, as mentioned in my first paper, also 
answer ; but I have found an objection in practice to this method, that it is difficult 
to hit the exact point at which moisture begins to deposit itself. Ether from its 
gradual evaporation, would enable the observer to be more accurate, with less at- 
tention . the former often requires several trials. 
Another conclusion of, perhaps, real importance, is derivable from the above ex- 
periment. I mean the great impurity of the ether used in this country as medicine. 
Its power, when pure, is so great as well as peculiar, that we can easily imagine 
a' mixture of alcohol and ether may fail to produce equivalent effects. This is a 
subject, however, which I must leave to your medical readers, merely suggesting, 
that if the view here indicated be worth attention, the test, already described, will 
always enable them to distinguish pure from alcoholic ether. This test has even a 
wider range of usefulness,— for it is equally applicable to the determination of the 
strength of alcohol or spirit, in every stage of dilution. This particular applica- 
tion of the thermometer I have long had in contemplation, and have been engaged in 
a series of experiments on the subject, from which I hope to show that it is one of 
the most convenient hydrometers that can be used ; and not inferior in accuracy, 
for any of the purposes required in commerce, to the more expensive and frequently 
troublesome instruments commonly employed. I hope to give you, ere long, the 
result of my meditations on this subject for publication in your work. 
I am, Mr. Editor, 
Your obedient Servant, 
D. 
