Properties of the Feldspars. 97 



strip, every draught of air, but most of all the judgment of 

 the observer as t;o when the substance appeared to melt, all 

 entered into the result to a very considerable degree. And 

 there is 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 

 quantities of a substance led us to study this method with 

 great care, and we were fortunate enough to possess an instru- 

 ment of Prof. Joly's own model made by Yeates & Son, 

 Dublin, but the results obtained with it, even under most favor- 

 able 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 cases 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 Hoi born and Day for the deter- 

 mination of the melting points of the metals, measured his 

 temperatures with thermoelements, and used several grams of 

 material in his determinations, 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 

 become a thin liquid. 



Detailed Plan. — 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 skillful 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 mineral to a liquid must of course be 

 defined by an absorption of heat. Whether the appearance of 

 the mineral charge in the furnace will offer a trustworthy 

 index through which to locate this absorption, may well be 

 expected to differ with different substances. ' Nearly all 

 observers have recorded the fact that many substances of this 

 class remain very viscous after melting and that the transition 

 is not well marked in the appearance of the material. 



