36 SCIENCE IN SHORT CHAPTERS. 



imitate the action of freezing without the aid of natural or 

 artificial frost. Epsom salts, nitre, alum, sulphate of iron, 

 Glauber's salts, etc., were tried. The last-named, Glauber's 

 salt (or sulphate of soda), which is very cheap, was found 

 to be the best for the purpose. 



His method of applying the test is as follows : Cut the 

 specimens into two-inch cubes, with fiat sides and sharp edges 

 and corners, mark each specimen with a number, either by 

 ink or scratching, and enter in a book all particulars concern- 

 ing it. Make a saturated solution of the sulphate of soda in 

 rain or distilled water, by adding the salt until no more will 

 dissolve ; perfect saturation being shown by finding, after 

 repeated stirring, that a little of the salt remains at the bottom 

 an hour or two after the solution was made. Heat this solu- 

 tion in a suitable vessel, and when it boils put in the marked 

 specimens one by one, and keep them immersed in the boiling 

 solution for half an hour. Take out the specimens separately 

 and suspend them by threads, each over a separate vessel con- 

 taining some of the liquid in which they were boiled, but 

 which has been carefully strained to free it from any solid 

 particles. In the course of a day or two, as the cubes dry, 

 they will become covered with an efflorescence of snow-like 

 crystals ; wash these away by simply plunging the specimen 

 into the vessel below, and repeat this two or three times daily 

 for four or five days or longer. The most suitable vessel for 

 the purpose is a glass " beaker," sold by venders of chemical 

 apparatus. 



In comparing competing samples, be careful to treat all 

 alike i.e. boil them together in the same solution, and dip 

 them an equal number of times at equal intervals. 



Having done this, the result is now to be examined. If the 

 stone is completely resistant the cube will remain smooth on 

 its surfaces and sharp at its edges and corners, and there will 

 be no particles at the bottom of the vessel. Otherwise, the 

 inability of the stone to resist the test will be shown by the 

 disfigurement of the cube or the small particles wedged off and 

 lying at the bottom of the liquid. Care must be taken not to 

 confound these with crystals of the salt which may also be 

 deposited. These crystals are easily removed by adding a 

 little more water or warming the solution. 



For strict comparison the fragments thus separated should be 

 weighed in a delicate balance, such as is used in chemical 

 analysis. 



