112 Dulong and Petit on the Measure of Temperatures, [FEB. 
I shall conclude this letter by informing you of a new mineral, 
very remarkable, on account of its composition. I have given 
it the name of Polyhalite. According to my analysis, 100 parts 
of it contain the following ingredients : 
Hydrous sulphate of ates) noisabimaiee OSU 
Anhydrous sulphate of lime. ........ 22:36 
Sulphate of potash ...... 400 oie we eh ORAS 
Anhydrous sulphate of magnesia .... 20°11 
Common salt. ..... $o.c'ee 6 pre wate ener yd 
Game OLITON, cide py dn: desaelione He 
99:20 
This mineral occurs in the beds of rock salt at Ischel, in 
Upper Austria, and has been hitherto erroneously considered by 
mineralogists as muriacite; and under the name of fibrous 
muracite, it has been described as a variety of that mineral 
substance. 
ARTICLE V. 
Researches on the Measure of Temperatures, and on the Laws of 
the Communication of Heat. By MM. Dulong and Petit.* 
Introduction. 
From the beginning of experimental physics, it has been 
perceived that of all the effects produced by heat, the changes 
of bulk which bodies undergo ought to be preferred to all the 
other phenomena due to the same cause to measure the natural 
or artificial changes of temperature. But there was a great 
distance between this first perception and the knowledge requi- 
site to. subject the construction of thermometers to invariable 
processes, which should render their indications comparable 
with each other. The frequent employment of these instruments, 
and the utility of the data which they furnish, have often drawn 
the attention of philosophers to all the circumstances that can 
contribute to their perfection. And ail these circumstances have 
been studied with so much care, and at such length, that nothing 
further remains to be desired relative to that object. 
Great precision was doubtless indispensable in thermometrical 
observations ; but this was not sufficient to lead to an accurate 
knowledge of the theory of heat. We might indeed refer all the 
phenomena to an arbitrary scale of temperature, and form empi- 
rical formulas, which should represent the observations with 
* Translated from the Ann, de Chim. et Phys. vii. 113. This memoir gained 
the prize voted by the Academy of Sciences in the public meeting of March 16, 
1818, ; 
