x Historical Sketch of the Physical Sciences, 1818. 
December (or at least the last two of them), in which it would 
be necessary for me to be employed in drawing up the paper, I 
am almost wholly occupied in teaching, and could not, if I were 
to make the attempt, spare sufficient time for so laborious a 
task. We have, therefore, after much consideration, adopted a 
plan, which bids fair to improve the value of these papers, while 
it does not interfere with my duties as a professor of chemistry— 
at least so seriously. The plan is to publish two supple- 
mentary numbers, each to be prefixed to its respective volume. 
In the first, or July supplement, we propose to give an histo- 
rical sketch of the progress of chemistry and mineralogy during 
the preceding year. This, I trust, I shall be able to draw up 
myself. The other supplement will contain a sketch of the 
progress of mechanical philosophy, botany, and zoology, during 
the preceding year, and will be drawn up by gentlemen well 
ualified to do justice to their several departments. Such is the 
plan which will be hereafter followed by the Editor of the Annals 
of Philosophy, and which it is hoped will meet with the appro- 
bation of its readers. 
I. CHEMISTRY. 
Several very important additions have been made to the science 
of chemistry during the course of the year 1818. In order to 
put my readers fully in possession of the facts, I shall be under 
the necessity of taking up some topics which came under our 
notice in the historical sketch printed in the Annals of Philosophy 
for July, 1818 ; but I shall take care to avoid all useless repeti- 
tion. The advantages of arrangement are so obvious that need 
make no apology for classing the different discoveries under 
their general heads. 
I. LIGHT AND HEAT. 
1. Measure of Temperatures.—All the precise notions respect- 
ing heat which we have acquired are derived from the use of 
the thermometer. The importance of this instrument has been 
long known, and much labour has been bestowed by philoso- 
phers in ascertaining the best way of graduating thermometers 
so as to make them comparable with each other. Now a ther- 
mometer is an instrament so contrived as to measure the dilata- 
tion of a liquid, and mercury has been found the most convenient 
liquid for the purpose. When heat is a plied to mercury, it 
increases in bulk, and, rising ina ple glass tube, indicates 
the degree of heat to which it is exposed. Several points remain 
still to be settled before the thermometer, even in its present 
improved state, can convey to us precise information. Do equal 
increments of heat occasion equal increments of bulk in mercury? 
Or do bodies expand more at high temperatures when they 
receive an equal increment of heat than at low temperatures ¢ 
Qr at what rate do they expand? These and several similar, 
