. IT IS ACTED ON BY LAMPLIGHT. 



837. Impressed with the importance of possessing, for the study of the properties of 

 the tithonic rajs, some means of accurate measurement, I have resorted in vain to many 

 contrivances ; and, after much labour, have obtained at last the instrument which it is 

 the object of this paper to describe. 



838. The tithonometer consists, essentially, of a mixture of equal measures of chlo- 

 rine and hydrogen gases, evolved from and confined 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 resulting from its union is removed by rapid 

 absorption. 



839. The theoretical conditions of the instrument are, therefore, sufficiently simple ; 

 but, when we come to put them into practice, obstacles which appear at first sight insur- 

 mountable 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 ra- 

 pidity through water into air. 



840. Without dwelling farther 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 hydro- 

 gen, 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 erro- 

 neous. The feeblest gleams of a taper produce a change. No farther proof of this 

 is required than the tables given in this chapter, in which the radiant source was an 

 oil-lamp. For speed of action, 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. 



841. Proofs of the Sensitiveness of the Tithonometer. The following illustrations 

 will show that the tithonometer is promptly affected by rays of the feeblest intensity, 

 and of the briefest duration. 



842. When, on the sentient tube of the tithonometer, the image of a lamp flame 

 formed by a convex lens is caused to fall, the liquid instantly begins to move over the 

 scale, and continues its motion as long as the exposure is continued. It does not 

 answer to expose the tube to the direct emanations of the lamp without 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. 



843. 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. 



844. The experimenter may farther assure himself of the extreme sensitiveness of 



