lO 



ATLAS OF ABSORPTION SPECTRA. 



of the quartz wedge. This was true for some dyes but for concentrated 

 solutions of certain other dyes (notably the sodium salt of /-methoxy- 

 toluene-azo-/5-naphthol-di-sulphonic acid) some compensating sj'stem was 

 absolute!}' necessary. Finally, the quartz wedge was made with the utmost 

 care by an expert optician, special pains being taken to have the edge 

 through d, perpendicular to the plane aicd, as sharply defined as possible, 

 and the surfaces whose traces are denoted by ad and dc were accurately 

 plane. 



(4) Figure 4 presents a side view and an end view of the vulcanite frame 

 into which the quasi-box just described fitted. This frame was shaped 



out of a single block of vulcanite, for 

 experience showed that a cemented 

 system of several pieces was not dur- 

 able; also a dielectric was needed to 

 keep the sparks from jumping to the 

 screws. P indicates a httle depression 

 Fig. 4 Four- fifths natural size. which fitted over the point of the 



screw T. P' designates the end of a straight line along which the rounded 

 extremity of the screw T' slid. P" is the cross-section of a shallow, 

 V-shaped groove along which the pointed end of the third screw, T", like- 

 wise slid. The perforations M, M', etc., correspond to each other and to 

 the associated wooden pegs mentioned above as (5). 



Figure 5 is an unconventional 

 sketch of the cell when completely 

 assembled. 



A cell of the construction just 

 described is very well suited to the 

 study of thin layers of solutions in solvents of relatively high boiling-points, 

 such as water and amyl alcohol, but, unless inclosed in some suitable vessel, 

 it is not applicable to solvents of lower boiling-points like ethyl alcohol, 

 ether, chloroform, etc. 



CEMENTS. 



A few words concerning cements may not be superfluous because a great 

 many receipts were tried and none was considered entirely satisfactory. 

 No single cement was found which satisfied the following three necessary 

 conditions : (a) Of being unaffected by hot or cold water ; {d) of being insolu- 

 ble in the alcohols, ether, chloroform, carbon bisulphide, etc. ; (c) of drying 

 or setting in three or four days, at most. 



The plan used by Prof. H. N. Morse, in waterproofing cells for the 

 study of osmotic pressure, gave the best results and hence it was followed 



Fig. 5. 



