on the Existence of a Fourth Imponderable. 107 



liminary observation should be made, and, if necessary, the 

 proper adjustments secured. 



Suppose now it were required to know how much blue light 

 is transmitted by a given solution. A trough is to be pro- 

 vided, which may be formed by drilling a hole two and a half 

 inches in diameter through a thick piece of plate glass; on 

 each side of this a thin piece of similar glass is laid, the trough 

 having been filled with the solution under investigation. 

 Troughs made in this manner never leak; they completely 

 answer their purpose, and are easily washed and refilled. 



Let the substance under trial be a concentrated solution of 

 bichromate of potash. Having adjusted the lamps and filled 

 the trough, set it before one of them, supporting it by a proper 

 foot. On looking through the tube of the photometer, if the 

 absorbent cell of the copper solution has been previously re- 

 moved, the circle of light will be seen very brightly illumi- 

 nated ; one of its halves of a yellow tint, but not much less 

 luminous than the other. The copper cell being now restored 

 to its place, on looking again through the tube there is a stri- 

 king contrast, one half of the circle is of a bright blue, but 

 the other seems totally black : with solutions which cut off the 

 blue rays less perfectly this blackness is of course less intense; 

 in these cases the lamps are to be moved into such positions 

 that the two halves of the circle are equally illuminated, and 

 its dividing diameter invisible. As the eye is not disturbed 

 with any difference of colour, the observation can be made 

 within small limits of error. The calculation of the relative in- 

 tensities can then be made by the common photometrical law. 



As this photometer is affected by rays of the same refrangi- 

 bility as those which affect the tithonometer, it is clear that if 

 it be the rays of light which are operative in the union of chlo- 

 rine and hydrogen, the results given by the two instruments 

 should correspond within certain small limits of error. 



With respect to the tithonometer, I have improved this in- 

 strument considerably ; by shading it with a glass case, which 

 cuts off thermometric disturbance ; by taking the observations 

 through a small telescope, which avoids parallax; by having 

 the scale moveable so as to slide along the tube ; by making 

 one charge of gas last for a great number of experiments j by 

 completing the tithonization before commencing; by altering 

 the position of the adjusting wire, so as to bring it nearly down 

 to the end of the tube. The detail of these changes would, 

 however, detain me now too long; they will be given at some 

 suitable time hereafter. 



I may, however, record as a striking fact, that so great is 

 the sensitiveness of chlorine and hydrogen, that a mixture will 

 actually explode by the rays of an electric spark. In this 



