Description of the Steam Pyrometer. 97 
on several accounts to be preferred. First we may always use the 
same amount of hot matter to produce the vapor, and consequently 
compare the actual heats of two melted masses without calculation. 
Second, the hot body may be directly applied to the water without 
the intervention of any enclosing vessel. Third, the pouring of the 
hot metal or liquid into water might not always be convenient or safe, 
as for example when the latter is of greater specific gravity than the 
former. When, for example, oil is laid at a very high temperature, 
on the surface of water, the sudden ebullition of the water, would 
be in danger of causing an explosion that would project the oil up- 
wards with great force. 
When we plunge a solid into a melting mass of metal, and allow it 
to remain for some time, it will acquire the temperature of the mass 
of melted matter, but the solid must have certain peculiar properties 
to fit it for this purpose. 
First, it must not melt at a lower point than that of the fluid which 
_ it is intended to test. 
Second, it must allow of being quenched in boiling water from the 
highest temperatures employed, without cracking, scaling, oxidizing, 
or undergoing any augmentation of weight by absorbing the liquid. 
Third, it must have as high a specific heat as practicable 
Fourth, it should be capable of being easily wrought ee the pe- 
culiar form required for the instrument with which it is to be used. 
Among the substances best adapted for the purpose are the follow- 
ing, against each of which the specific heat is marked together with 
the name of the author, whose determination has been followed. 
Spe. Heat. Authorities, 
Crown glass, .2000 rvine, 
White glass, -1870 Wilcke. (.1770, Pet. and Dul.) 
White clay, bu -1850 Gadolin. 
Black lead or na -1830 
White cast iron, .1320 Do. 
Soft bar iron, sp. gr. 7.724, .1190 Do. 
Platinum, .0314 Petit and Dulong. 
The chief parts of this instrument are a boiler, A; (Fig. 1.)—a 
stand, S ;—a-balance beam, D, for weighing the baller and its con- 
tents ;—a lamp, L, to heat the water and to maintain ebullition be- 
tween experiments ;—a receiver, R, (Figs. 2 and 3.) and a cylinder 
of metal, 1, to be employed as a standard. 
The boiler is formed of two concentric cylinders of copper. The 
inner cylinder is two and a half inches in diameter, the exterior one 
Vou. XXI.—No. 1 1 : 
