274 ELEMENTARY CHEMICAL MICROSCOPY 



begins to recrystallize at once. Examine between crossed nicols under the micro- 

 scope, noting well the phenomenon which takes place. As soon as the transition 

 is complete, inoculate the edge of the solidified mass with a crushed crystal of 

 monochloracetic acid taken from the bottle. A third transformation will now be 

 observed. Monochloracetic acid exists in three modifications: a, a stable form 

 having a melting point of 61-62 , crystallizing in needles and prisms; /3, a meta- 

 stable form with a melting point of 55— 56 ; and 7, also a metastable form melting 

 at 50-51 °; (3 and 7 crystallize in rhombs. All three forms are monoclinic. When 

 the a form is melted and suddenly cooled 7 crystals are formed. 7 crystals when 

 scratched transform into /3 and the /3 crystals inoculated with a crystals, change 

 at once to the a form. Occasionally 7 crystals pass at once into the a modifica- 

 tion without first changing into /3. 1 



13. Place a drop of olive or cotton seed oil upon a slide, introduce a small quantity 

 of stearic acid, cover, heat until the stearic acid melts. Cool and examine. Use 

 crossed nicols. 



14. Place a small fragment of fresh butter on a slide, press down a cover-glass 

 until a very thin layer is obtained. Examine under the microscope. Examine 

 between crossed nicols and with a selenite plate in proper position. 



Heat the preparation very gently, cool and again examine. Examine with crossed 

 nicols and a selenite. 



Test a sample of " process " butter and one of oleomargarine, without melting. 



The Influence of Jelly-like Material and Gums upon Crystallization. 



15. Prepare drops of saturated aqueous solutions of several salts readily forming 

 well defined crystals. Use for this purpose some of the salts already studied and 

 sketched. 2 Warm these drops and add a drop of a warm solution of gelatine. Mix 

 thoroughly and warm gently; set aside to cool and crystallize. Examine, sketch 

 and describe the crystals. 



16. Repeat Experiment 15, substituting for the gelatine a concentrate solution 

 of gum arabic. 



17. Dissolve in a large drop of 10 per cent gelatine a little potassium arsenate. 

 Spread the drop so as to have a thin layer about 5 mm. in diameter and 1 mm. 

 thick. Set aside to cool and when cold and the gelatine has set, place at the center, 

 a drop of silver nitrate acidified with nitric acid. Rhythmic crystallization will 

 take place and the silver arsenate formed will appear in concentric rings with 

 alternate clear spaces (Liesegang's rings). It may be necessary for the student 

 to make several attempts before a really satisfactory preparation is obtained. 

 Similar phenomena may be obtained with potassium bichromate and silver nitrate. 

 The cause of this periodic crystallization is not yet clearly understood. 



Crystals formed by Sublimation. 



18. Place a small quantity of Phthalic anhydrid in a small watch glass; cover 

 with a cold object slide; heat gently over the " micro " flame of the Bunsen burner, 

 employing the clamp described and illustrated on page 294 for holding watch glass 



1 Mier and Isaac: Phil. T. Roy. Soc. 209 (1909) 337. Barker, T. V.: Practical 

 Suggestions towards the Study of Crystals. Oxford, 1921. 



2 The following will be found interesting. Copper sulphate; ammonium sul- 

 phate; ammonium nickel nitrate; mercuric chloride. 



