DIFFERENTIAL PRESSURE ON MINERALS AND ROCKS 493 



Alum, if employed, can readily be removed at the conclusion of 

 the experiment by placing the deformed tube in hot water on a water 

 bath for a short time. But it has the disadvantage when used as an 

 embedding material for limestones, that, whether in a state of fusion 

 or solution, it attacks carbonate of lime to a noticeable extent. On 

 soaking out the contents of the tube with warm water at the conclusion 

 of the experiment, however, a very distinct effervescence always 

 ensues and this is especially marked if the marble has been rendered 

 at all pulverulent. The amount of calcite which is thus dissolved is 

 not, however, great but it is quite sufficient to etch the surface and 

 destroy the polish of the marble or the transparency of the calcite 

 crystal employed in the experiment. In the case of all the more 

 resistant minerals and rocks, this objection of course does not exist. 



When sulphur, fusible metal, or paraffine wax is used, these are 

 removed at the close of the experiment by simply heating the tube in 

 a deep porcelain dish, over which a second smaller one is inverted on 

 a sand bath or a water bath as the case may be. 



In order to get some clear idea of the resistance to deformation 

 offered by these several embedding materials under the experimental 

 conditions, a series of experiments was carried out on the deforma- 

 tion of copper tubes or collars of the size usually employed, some of 

 which were left empty while others were filled respectively with the 

 several embedding materials referred to above. 



The following table shows the results obtained in tabular form. 

 The values are given in pounds and each represents the mean of two 

 closely concurrent experiments. 



It is seen in the first place that the copper tube when filled with 

 paraffine wax offers but very little more resistance to deformation 

 than does the empty tube. Under the conditions of the experiment 



