182 Dr. Thomson on Oxymuriate of Lime. [Marcu, 
augmented by the entrance of cold mercury from the stem of 
the thermometer. But the volume of mercury being known, 
and likewise its temperature, it was easy to estimate exactly the 
amount of this correction, which, though very small, ought not 
to have been neglected. 
Such is the mode which we always followed, in conduct- 
ing and calculating all our experiments. We satisfied ourselves 
with determining the velocity of cooling for excesses of temper- 
ature, differmg from each other by 20 degrees. And that we 
might not make this memoir too tedious, we have withheld all 
the intermediate calculations which led to our determinations. 
We shall now enter upon a detail of our experiments, stating 
them in the order in which they were made. 
Our preliminary researches having made us acquainted with 
the influence of the nature of the surface upon the law of cooling, 
it was necessary to study that law under different states of the 
surface of our thermometers. But it was necessary likewise that 
these surfaces should not experience any alteration from the 
highest temperatures to which they should be exposed. The 
only two which appeared to us to answer this condition are 
surfaces of glass and of silver. Accordingly most of our expe- 
riments were made, first preserving to the thermometer its 
natural surface, and then covering it with a very thin leaf of 
silver. These two surfaces possess, as is known, very different 
radiating powers ; glass being one of the bodies which radiate 
most, and silver of those which radiate least. The laws to which 
we have arrived, by comparing the cooling of these two surfaces, 
are of such simplicity that there can be no doubt of their being 
applicable to all other bodies. 
(To be continued. ) 
Articie II. 
On Mr. Tennent’s Bleaching Salt ; known by the Name of Oxy- 
muriate of Lime. By Thomas Thomson, M.D. F.R.S. 
Wuen I was drawing up the fifth edition of my System of 
Chemistry, one of the substances, respecting which I found 
myself unable to form a definite opinion, was the bleaching salt, 
originally invented by Charles Tennent, Esq. and well known in 
commerce under the name of orymuriate of lime. 1 found myself, 
therefore, obliged to omit the substance altogether, resolving, 
however, to ascertain its nature as soon as I should have sufh- 
cient leisure for that purpose. I got a quantity of it accordingly 
from Mr. Tennent, Jast autumn, quite fresh, and subjected it to 
the requisite experiments. I shall in this paper give a short 
sketch of the principal facts which I observed, reserving a more 
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