BRIDGMAN. — MERCURY UNDER PRESSURE. 403 



provided time enough is allowed. There are also other effects of some- 

 what the same general character, such as hysteresis, not usually in- 

 cluded in the name. But there will be included in this discussion any 

 effect in virtue of which the strain is not a single valued function of 

 the stress only. The general nature and interpretation of these ef- 

 fects is still obscure, but there are a few facts which are well known. For 

 instance, in all ordinary testing of materials it is first necessary to ac- 

 commodate the metal to the particular stress conditions of the moment. 

 The strain under a given stress is not the same the first time the stress 

 is applied as the second, and not until the application of the stress a 

 number of times does the strain come to depend always in the same 

 way on the stress. Even after complete accommodation has been at- 

 tained, the material may continue to show elastic after-effects, that is, 

 continued yield after the application, or continued recovery after the 

 release or stress. Or it may also show hysteresis. These effects be- 

 come increasingly important as the magnitude of the stress is increased, 

 or as the character of the stress changes from one uniform throughout 

 the mass to one changing from point to point. The effects become 

 most important of all when the material itself has become non- homo- 

 geneous by the exceeding of the elastic limit in some places while the 

 remaining parts are without permanent deformation. 



Various effects of just this kind are to be expected in these exper- 

 iments, and may perhaps be fairly large in comparison with the usual 

 magnitude of the effects, because of the wide range of stress here em- 

 ployed. It should be pointed out that one advantage of this method, 

 namely, that the elastic deformation of the vessels does not enter the 

 results, is in part offset by the various effects of the kind mentioned 

 here, because in virtue of them the volume of the containing vessel 

 may change while freezing or melting is going on at constant pressure. 

 Furthermore, these effects, which are themselves intrinsically small, 

 may appear with magnified effect in the result. Thus the actual 

 change of volume of the mercury during freezing is about 1 per cent of 

 the total internal volume of the containing vessel, so that a change of 

 volume of 1/100 per cent of the containing vessel produced by elastic 

 after-effects during freezing will appear in the result magnified to 1 per 

 cent. As a matter of fact, there were found discrepancies between the 

 values under increasing and decreasing pressure amounting in the worst 

 case to 3 per cent. 



It was possible, however, by an examination of the discrepancies 

 themselves and by a brief consideration of the method of experiment to 

 select from these the most probably accurate results. During the 

 course of an experiment, the stress history of the steel in its bearing on 



