some Copper Salts in Aqueous Solution. 325 



more complex molecular aggregates in the solution. On 

 diluting a strong solution of copper chloride, he found a 

 sudden change, at a certain point, in the width of the 

 penumbra, due to the breaking-up of the complex molecules. 

 Table IV. (p. 324) contains the measurements of the breadth 

 of the penumbra, calculated from Table I., for a number of 

 solutions of copper chloride. 



On the whole the numbers cannot be regarded as showing 

 any sudden diminution in the breadth of the penumbra. The 

 variations appear to be accidental, and are probably due to 

 variations in the brightness of the spectra caused by alterations 

 in the relative positions of the lamp and the tubes containing 

 the solutions. 



Photometric Results. 



The method of Vierordt was employed, the instrument used 

 being a universal spectro-photometer by A. Kriiss of Hamburg. 

 As this instrument has already been described elsewhere *, no 

 lengthy description of it will be necessary. The apparatus 

 consists of an ordinary spectroscope, the slit of which is 

 divided into two halves, each of which opens symmetrically, 

 and in an accurately measurable manner, by means of micro- 

 meter-screws, the heads of which carry circles divided into 

 100 parts. By this means it is possible to vary the brightness 

 of the upper and lower halves of the spectrum independently 

 of each other by varying the width of the slits. A second 

 slit in the eyepiece of the observing-telescope makes it 

 possible to cut out a band in any part of the spectrum. 

 The position of the centre and of the edges of this slit 

 (which also opens symmetrically) can be read off by means 

 of micrometer-screws. 



The ocular slit was opened 56 divisions (on its screw), 

 which corresponds to a breadth of about 0'27 milliin. This 

 divided the whole length of the visible spectrum into 19 

 parts, in each of which measurements were made. 



For an account of the precautions which must be taken in 

 using the instrument it will be sufficient to refer to the works 

 of Gr. and H. Kriiss and of Yierordt, more especially Die 

 Anwendung des Spectralapparates zur Photometric der Ab- 

 sorptions- spectren (Tubingen, 1873). 



The solutions were contained in cells of various lengths. 

 As the measurements are most accurate when the absorption 

 lies between 50 and 85 per cent, of the total light, the length 



* Berichte, xix. p. 2739 (1886); Zeitsclmft anal Chan. xxi. p, L82 

 (1882) ; G. & H. Kriiss, Kolorimetrie, p. 90 et seq. 



