96 Description of the Steam Pyrometer. 
Arr. VI.—Description of an instrument called the Steam Pyrome- 
ter; by Water R. Jonnson, Professor of Mechanics and Nat- 
ural Philosophy, in the Franklin Institute, Philadelphia. 
A careful attention to guard the containing vessel in which we 
produce steam from boiling water by means of metal, or other solid 
or liquid bodies capable of being heated in open vessels above 
212° Fah. will enable us to measure with great accuracy, the quanti- 
‘ty of heat which such solid or liquid body expends in cooling, from 
the temperature at which it is first put in, down to the boiling point 
of water. : 
The mode of calculating the temperature when the specific heat 
is known, has already been given.* The only pointsof much diffi- 
culty in rendering the formula heretofore stated, directly useful in 
pyrometry are, 1, the necessity of defending the vessel in which the 
steam is produced, from the effects of radiation and conduction dur- 
ing the operation; 2, the obviating of loss in transferring the hot 
body to the liquid through the air; 3, the means of obtaining and 
marking the true boiling point, and 4, the means of speedily and ac- 
curately weighing the liquid, and showing how much has been evapo- 
rated during an experiment. 
To these causes of inconvenience, may be added, that which results 
from the low specific heats of some of the substances, to be employ- 
ed as standards.—Such are several of the metals as platina, gold, 
&c. - It is obvious that the method of plunging the body of which we 
would know the temperature directly into boiling water, can be 
adopted only with regard to:solids, which remain. unchanged after 
being quenched in water, and which are not capable of imbibing the 
fluid, on account of porousness, or such physical characters as would 
render them liable to combine chemically with the water. 
When we have to deal with liquids of which the temperatures extend 
beyond that of boiling mercury, that is, of mercury boiling in vacuo, 
(which must necessarily limit our use of the mercurial thermomete?;) 
we must either pour such liquid into the boiling water, if a melted metal 
which will not undergo change in that method of cooling, or must en- 
close it in a suitable vessel extremely thin and of materials to sustain the 
action of water upon it, or must immerse in the hot liquid or the melted 
metal, a mass of some other matter capable of preserving its form 
under a heat greater than that of the liquid. The latter method is 
* See American Journal, Vol. XX. p. 316, July 1831. 
