192 



CONSTRUCTION OF THE INSTRUMENT. 



TABLE III. 

 Showing that the Maximum for the Tithonometer is in the Indigo Space of the Spectrum. 



In^this table the spaces are equal ; the centre of the red, as insulated by cobalt blue 

 glass, is marked as unity ; the centre of the yellow, insulated by the same, being marked 

 3 ; the intervening region being divided into two equal spaces, and divisions of the 

 same value carried on to each end of the spectrum. 



859. As instruments will no doubt be hereafter invented for measuring the phenom- 

 ena of different classes of rays, it may prove convenient to designate the precise ray to 

 which they apply. Perhaps the most simple mode is to affix the name of the ray itself. 

 Under that nomenclature, the instrument described in this paper would take the name 

 of indigo-tithonometer. 



860. There is no difficulty in adapting this instrument to the determination of ques- 

 tions relating to absorption, reflexion, and transmission. Thus, I found that a piece ot 

 colourless French plate-glass transmitted 866 rays out of 1000. 



861. Description of the Instrument. First, of the Glass Part. The tithonometer 

 consists of a glass tube bent into the form of a siphon, in which chlorine and hydro- 

 gen can be evolved from muriatic acid, containing chlorine in solution, by the agency 

 of a voltaic current. It is represented by Jig. 105. where a b c is a clear and thin tube, 

 four tenths of an inch external diameter, closed at the end, a. At d, a circular piece 

 of metal an inch in diameter, which may be called the stage, is fastened on the tube, 

 the distance from d to a being 2'9 inches. At the point x, which is two inches and a 

 quarter from d, two platina wires, x and y, are fused into the glass, and, entering into the 

 interior of the tube, are destined to furnish the supply of chlorine and hydrogen ; from 

 the stage, d, to the point b, the inner bend of the tube is 2-6 inches, and from that 

 point to the top of the siphon, c, the distance is three inches and a half. Through 

 the glass at z, three quarters of an inch from c, a third platina wire is passed ; this wire 

 terminates in the little mercury cup, r, and x and y in the cups p and q respectively. 



862. Things being thus arranged, the instrument is filled with its fluid, prepared as 

 will presently be described ; and as the legs, a b, b c, are not parallel to each other, but 

 include an angle of a few degrees, in the same way that Ure's eudiometer is arranged, 

 there is no difficulty in transferring the liquid to the sealed leg. Enough is admitted 

 to fill the sealed leg and the open one partially, leaving an empty space to the top of 

 the tube, at c, of two and three quarter inches. 



863. A stout tube, six inches long and one tenth of an inch interior diameter, ef, is 

 now fused on at c. Its lower end opens into the main siphon tube ; its upper end is 

 turned over &tf, and is narrowed to a fine termination, so as barely to admit a pin, 

 but is not closed. This serves to keep out dust, and in case of a little acid passing out, 

 it does not flow over the scale and efface the divisions. At the back of this tube a 



