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 describe. 



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, notwithstanding the contraction 

 which may be going on in its volume, and the muriatic acid re- 

 sulting from its union is removed by rapid absorption. 



The theoretical conditions of the instrument are therefore suf- 

 ficiently simple ; but, when we come to put them into practice, 

 obstacles which appear at first sight insurmountable are met with. 

 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- 

 ity through water into air. 



Without dwelling further on the long catalogue of difficulties 

 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 a change. 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 tithonographic com- 

 pound can approach it; alight, which perhaps does not endure 

 the millionth part of a second, affects it energetically, as will be 

 hereafter shown. 



Proofs of the sensitiveness of the Tithonometer. — The follow- 

 ing illustrations will show that the tithonometer is promptly af- 

 fected by rays of the feeblest intensity, and of the briefest du- 

 ration. 



