fees 
218 Prof. Draper’s Description of the Tithonometer. 
urement, I have resorted in vain to many contrivances; and, af- | 
ter much labor, have obtained at last the instrument which it is © 
the object of this paper to describ 
The tithonometer consists essentially of a mixture of equal | 
measures of chlorine and hydrogen gases, evolved from and con- 
fined by a fluid which absorbs neither. This mixture is kept in 
a graduated tube, so arranged that the gaseous surface exposed to 
the rays never varies in extent, a" the contraction 
which may be going on in its volume, and tl muriatic acid re- 
sulting from its union is removed by rapid absorp tion. 
The theoretical conditions of the instrument are ‘therefore suf- 4 
tacles which appear at first sight insurmountable are met will : 
The means of obtaining chlorine are all troublesome; no liquid 
is known which will perfectly confine it; it is a matter of great — 
difficulty to mix it in the true proportion with hydrogen, and © 
have no excess of either. Nor is it at all an easy affair to obtain 
pure hydrogen speedily, and both these gases diffuse with rapid- 4 
ity through water into air. 
Without dwelling further on the long catalogue of difficulties q 
which is thus to be encountered, I shall first give an account of © 
the capabilities of the instrument in the form now described, | 
which will show to what an extent all those difficulties are al- 
ready overcome. In a course of experiments on the union of 
chlorine and hydrogen, some of which were read at the last 
meeting of the British Association, 1 found that the sensitiveness _ 
of that mixture had been greatly underrated. The statement 
made in the books of chemistry, that artificial light will not af- 
fect it, is wholly erroneous. The feeblest gleams of a taper pro- 
duce achange. No further proof of this is required than the — 
tables given in this communication, in which the radiant source _ 
was an oil lamp. For speed of action no tithonographie com-— 
pound can approach it; a light, which perhaps does not endure 4 
the millionth part of a ri aflects-it energetically, as will be — 
gepenticr shown. 
ing illustrations will show that the tithonometer is promptly af- 
fected by rays of the ren intensity, and of the briefest du- 
ration. 
See (ae ees 
Proofs of the sensitiveness of the Tithonometer—The follow- — 
