]889.] A. Vedlev— Volatility of some of tie Oompounds of Mercury. 193 



mei'cuiy commenced to form on the surface of the caustic soda, and thia 

 gradually increased until the whole surface of the caustic soda and part 

 of the interior surface of the glass was covered with a grey film. The 

 experiment was allowed to go on till Juno 10th, 1887, when the tube 

 was opened and examined. The mercury salt in the lower end of tho 

 tube was weighed, and found to be 10-8628 grams and had therefore lost 

 0'0132 gram. The mercury salt was examined, and was found to have 

 been partly reduced to mercurous chloride (calomel), and 0-018 gram of 

 this salt had been produced. This calomel when examined under tho 

 microscope was of a distinct ci'ystalHne nature, and crystals belonging to 

 the usual form for calomel* were found ; these crystals also answei'ed 

 to all the tests for calomel. No free mercury was found in the coiTosive 

 sublimate. 



Tho amount of mercury which had been deposited in tho upper 

 part of the tube, and which was due to the decomposition of tho vapour 

 of the mercuric chloride was also estimated, and was found to weigh 

 0-0112 gram, or very nearly the amount which would bo calculated from 

 the loss of weight, etc. of the mercuric chloride. It is clear therefore that 

 mercuric chloride is very decidedly volatile at ordinary air temperatures 

 in vacuo and when exposed to sunlight, and from a nearly circular sur- 

 face of the powdered salt about f inch in diameter a very appreciable 

 weight of mercuric chloride had evaporated and had passed up a tnbo 

 nearly six inches in length, while a further appreciable weight of iho 

 mercuric chloride had changed into mercurous chloride. 



To test whether this decided volatility was in any way facilitated 

 by the strong light used in the experiment, another tube | inch diameter 

 had been similarly prepared, but had been kept in the dark. This ex- 

 periment was started on April 19th, 1887. The action in this oaso 

 did not commence so rapidly as in the previous case. After a month 

 only about J of an inch of the lower end of the stick of caustic soda 

 was covered with tho deposit, and tho deposit gradually but slowly in- 

 creased. The tube was opened on January 2nd, 1889, and the incrusta- 

 tion was found of a yellow to a yellow brown colour. Tho deposit was 

 examined under the microscope, and no metallic mercury could bo 

 detected. The deposit was weighed, and found to be only 0-0010 gram., 

 and it was found to consist practically entirely of mercuric oxido 

 (HgO). 



Thus it is clear that the mercuric chloride is volatile at ordinary 

 air temperatures, but the volatility appears to bo increased by the direct 

 action of light. The reactions in the two cases are also slightly different. 



* Walts's Diet.. Chem. Vol. II, P. 135 fig. 237. 



