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differences several times greater than 5 appeared in our observations 

 with the Joly apparatus, unless the grains were prepared with the 

 greatest care and all the observations made by the same observer. 

 The size and form of the grains, the care used in locating them 

 exactly in the middle of the strip, every draught of air, but most of 

 all the judgment of the observer as to when the substance appeared 

 to melt, all entered into the result to a very considerable degree. 

 There is also another source of error with which we afterward became 

 familiar, which may serve to account for the very large differences 

 between Joly's results and our own later values with some of the 

 well-known minerals, though not with all. In certain of the minerals, 

 after melting, the resistance to change of' shape, due to viscosity, is of 

 the same order of magnitude as that due to the rigidity of the crystal 

 just before melting, a fact which may well have led to large errors of 

 judgment in this method of detecting melting points. 



The possibility of working very expeditiously with minute quanti- 

 ties of a substance led us to study this method with great care, and we 

 were fortunate enough to possess an instrument of Professor Joly's 

 own model, made by Yeates & Son, Dublin, but the results obtained 

 with it, even under most favorable conditions, are more in the nature 

 of personal estimates than of exact measurements of the change of 

 state. Its value for qualitative study, and in eases where only a very 

 minute quantity of a substance is available, is unquestioned. 



Doelter has employed electric furnaces, modeled after that in use at 

 the Reichsanstalt by Holborn and Day, for the determination of the 

 melting points of the metals. He measured his temperatures with 

 thermoelements, and used several grams of material in his determina- 

 tions, but he also judged of the approach of the melting point by the 

 appearance of the charge and usually recorded two temperatures the 

 first approach of viscous melting and the point where the material 

 appeared to have gone over into a thin liquid. 



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We determined from the first to get rid of this personal factor. 

 However carefully such observations may be made, and however well 

 supported by the reputation of a particular scientist for skilful and 

 exact work, they can not have a permanent value. Melting points of 

 pure minerals are not different, in principle at least, from the melting 

 points of other chemical compounds or of metals. They occur at less 

 accessible temperatures and involve some complicating phenomena, 

 as we shall see presently, but the change of state of a solid crystalline 



