APPENDIX. 387 



by the density of a cloudy precipitate. Though this book does not contain an 

 example of this method of analysis, it is of growing importance, being valuable 

 when only very small amounts of material are available. It is also supplied, 

 if desired, with an excellent lamp house, so that it can be used at night. In 

 fact, it is more satisfactory to use artificial light, since by means of mirrors 

 the illumination of the two fields can be made exactly equal. 



The instrument is shewn in fig. 54, and it will be seen that the plungers 

 are fixed and the cups movable. This does away with the space between the top 

 of the cup and the prism, which is apt to allow an indefinite amount of light 

 through in a Duboscq. The prisms are enclosed and so remain free from dust. 

 The use of dark glass for the sides of the cups and the plungers, the absence 

 of cements, improved mechanical arrangements for adjusting the zero and 

 moving the cups, and the reduction in the volume of fluid necessary, all 

 combine to make the instrument nearer perfection than anything yet 

 introduced.* 



The calculations necessary in colorimetric work are very simple. The 

 assumption is that the depth of colour is proportional to the concentrations 

 of the substance in the standard and in the unknown. Also that the depth 

 of field of the solutions that must be taken to get equality of tint vary in- 

 versely as the intensity of the colour produced, and therefore as the concen- 

 trations in the standard and unknown. So if the standard contains a certain 

 amount of material in a given volume and the unknown is made up to the same 

 volume, then 



Amount in standard Reading of unknown 

 Amount in unknown ~~ Reading of standard 



* Both instruments can be obtained from Messrs. Baird and Tatlock 

 (London). 



