402 Professor Draper's Description of the Titlwnometer. 



exposed to the rays never varies in extent, notwithstanding 

 the contraction which may be going on in its volume, and the 

 muriatic acid resulting from its union is removed by rapid 

 absorption. 



The theoretical conditions of the instrument are therefore 

 sufficiently simple; but, when we come to put them into prac- 

 tice, obstacles which appear at first sight insurmountable are 

 met with. The means of obtaining chlorine are all trouble- 

 some; 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 rapidity through water into air. 



Without dwelling further on the long catalogue of difficul- 

 ties which is thus to be encountered, I shall first give an ac- 

 count of the capabilities of the instrument in the form now 

 described, which will show to what an extent all those diffi- 

 culties are already 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, I found that the 

 sensitiveness of that mixture had been greatly underrated. 

 The statement made in the books of chemistry, that artificial 

 light will not affect it, is wholly erroneous. The feeblest 

 gleams of a taper produce 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 ac- 

 tion no tithonographic compound can approach it; a light, 

 which perhaps does not endure the millionth part of a second, 

 affects it energetically, as will be hereafter shown. 



Proqfsqf the sensitiveness of the Tithonometer.-*— Thefollowing 

 illustrations will show that the tithonometer is promptly affected 

 by rays of the feeblest intensity, and of the briefest duration. 



When, on the sentient tube of the tithonometer, the image 

 of a lamp formed by a convex lens is caused to fall, the liquid 

 instantly begins to move over the scale, and continues its mo- 

 tion as long as the exposure is continued. It does not answer 

 to expose the tube to the direct emanations of the lamp with- 

 out first absorbing the radiant heat, or the calorific effect will 

 mask the true result. By the interposition of a lens this heat 

 is absorbed, and the tithonic rays alone act. 



If a tithonometer is exposed to daylight coming through a 

 window, and the hand or a shade of any kind is passed in 

 front of it, its movement is in an instant arrested; nor can the 

 shade be passed so rapidly that the instrument will fail to 

 give the proper indication. 



