Camphor towards the Light. 365 



upon a shelf hi a dark cupboard. No crystals were formed after 

 many days ; on account of the enclosed space becoming saturated 

 with moisture, and thus preventing evaporation. To prove that 

 the absence of light had nothing to do with the result, a capsule 

 containing a few lumps of caustic potash was placed by the side 

 of the solution, and the whole was covered with a bell-glass. In 

 the course of a few hours the creeping crystals began to form on 

 the side of the vessel not covered with the glass plate, just as in 

 the case of similar solutions exposed to the light. Identical re- 

 sults were obtained when solutions of sulphate of zinc and of 

 sulphate of copper were placed in the dark enclosed space with- 

 out being covered with the bell-glass, provided the air were kept 

 dry by means of caustic potash. 



These experiments also serve to explain some facts which 

 puzzled Dr. Draper. A bottle of camphor in the dark produces 

 no deposit, even though left for months, but when put in the 

 window, it forms a deposit immediately. One bottle put within 

 another forms no deposit, even though exposed to the diffused 

 light of day. Exactly so. A bottle shut up in the dark is pro- 

 tected from radiation : it is equally warm all round, and, though 

 filled with vapour, there is no sufficient reason why a deposit 

 should be made at one part of the bottle more than another. 

 Put that bottle in the window, and it becomes cooled on the side 

 nearest the window-pane, and a deposit is made. Light has 

 nothing to do with this result. The experiment can be made in 

 the dark as well as in the light; indeed much better, for by 

 night there is a greater difference between the inner and the 

 outer temperatures than by day. I have taken a bottle of 

 camphor out of a dark cupboard, and placed it in the window 

 on which the moon was shining. A deposit was made within 

 three minutes on the side nearest the moon. Anyone satisfied 

 with the result of one experiment, would say that the light of 

 the moon produced the deposit. I have tried the same experi- 

 ment when there was no moon, and obtained the same result. 

 So also if a tube containing a little crude camphor be passed 

 through a cork into the centre of an empty bottle, there will be 

 no deposit on the tube on exposure to the light, because the tube 

 is equally warmed all round. So also if a camphor bottle be sur- 

 rounded with water at 100° or 110° and left to cool, there will be 

 no deposit. In an experiment of this kind, a quart glass jar was 

 three parts filled with water at about 110°, and an eight-ounce 

 phial, containing crude camphor that had been kept in the dark 

 some months without showing any signs of deposit, was plunged 

 into it. The temperature was now 100°; the jar was covered 

 with a large air-pump receiver and left facing the light. Next 

 morning the bottle was taken out and carefully examined ; there 



