HOW TO PHOTOGRAPH 



of — balsam and benzole, or a solution of 

 shellac in alcohol. The latter is much the 

 better of the two. Tiie solution should be 

 quite limpid. For soft limestones a soak- 

 ing of two or three days will be sufficient, 

 but a piece of boiler incrustation will re- 

 quire to be left in the solution for at least a 

 fortnight. When the rock is thoroughly im- 

 pregnated with the hardening solution, take 

 it out, and put it to dry in a warm place 

 until the solvent has evaporated, leaving the 

 balsam or shellac in the pores quite hard. 

 Sections of rocks which contain organisms 



— such as foraminifera, should not be 

 ground very thin, or most of the fossils will 

 be ground away, leaving the section quite 

 useless as a specimen. 



Most thin rock sections photograph bet- 

 ter by polarized light, as the structure is 

 thus much better shown. Sedimenta r 

 rocks, even white limestones, stop a great 

 deal of ligh', and v ill require a long expo- 

 sure. Such sections are always much im- 

 proved by a few days' soaking in balsam 

 and benzole, to render them more transpar- 

 ent. 



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CRYSTALS OF ARSENIC, FROM A DIPOSIT OF 002 GRAIN. 



Lesson XII. — Preparing Crystalliza- 

 tions FOR Photo- MICROGRAPHY. 



Most chemical crystals are mounted in 

 balsam, as they are usually viewed by polar- 

 ized light ; but where this is inapplicable, 

 or when the crystals are soluble in balsam, 

 this medium must not be employed, and the 

 crystals must be simply evaporated from an 

 aqueous or alcoholic solution on a clean 

 glass slide, and covered with a circle of thin 

 glass. The majority of chemical salts are 

 soluble in v/ater, and when typical crystals 

 are not required, the addition of a little 



gelatine to the water will usually produce 

 larger, and in some cases more beautiful 

 forms. However, as the genuine typical 

 forms will be generally required in a pho- 

 tograph, it is best to use nothing but dis- 

 tilled water, or pure alcohol, in making up 

 the solutions. To obtain typical crystals, 

 the solutions should not be concentrated, 

 but normal, and the evaporation of the salt 

 should not be accelerated by too much heat. 

 As a general rule, the most perfectly formed 

 crystals are obtained by very slow evapora- 

 tion in a cool place. Many salts, if evapo- 

 rated in a hot room, or over a lamp, give 



