184 Mr. Carl Barus on the Fusion 



in proportion as the furnace cools slowly. The latter con- 

 dition is met at least so long as glass is liquid, and data for 

 the former are fully given below. To avoid strains of dilata- 

 tion it would be desirable to make the glass solidify from the 

 bottom upward. This, however, my furnace as yet fails to do. 

 Indeed, I have frequently noted lateral holes shaped like an 

 inverted funnel and terminating in the otherwise smooth 

 surface of the meniscus. This may indicate the occurrence 

 of gas bubbles under the free surface or be a strain effect, 

 § 3. The tendency of these nearly unavoidable difficulties is 

 to make the solidification contraction too small ; and I have 

 therefore not been disturbed by them. 



Only at very high temperatures (above 1500°) did I obtain 

 apparent evidence of the viscosity of platinum relatively to 

 the pressure of the column of 25 centim. of molten rock, 

 whereas the combined system of platinum and clay tube with- 

 stood this pressure. There is also a slight deepening of the 

 meniscus from day to day, and a similar decrease of the 

 length of the platinum fusion-tube ; but, thus far, I have 

 encountered no serious error due to the high temperature 

 viscosity of the vessel, § 18. 



The assumption that the meniscus remains similar to itself 

 in form at all temperatures cannot be quite true. At white 

 heats, however, the glass wets platinum so thoroughly that 

 convex forms of meniscus need never be apprehended. It is 

 futile, however, to endeavour to make allowance for meniscus ; 

 but data for the difference between the (cold) bulk and micro- 

 metric measurements are given below. Although too much 

 reliance must not be placed on the behaviour of the solid 

 state, yet the values are redeemed from the character of mere 

 estimates by the close coincidence of the expansion of pla- 

 tinum and its solid glass core. Strains imposed on both the 

 metal and the glass do not probably exceed the limits of 

 elasticity of either, §§ 17, 18. When cooling is conducted 

 slowly enough, the experiments show that the platinum tube 

 is not dragged along seriously by the solidifying magma ; and 

 the data thus retain a degree of trustworthiness greater than 

 was anticipated, even in the solid state. At the same time 

 dilatational strain is reduced to a minimum, and constancy of 

 temperature throughout efficient parts of the furnace is 

 promoted. 



One error much in my way thus far has been the allowance 

 to be made for the expansion of the platinum probe dd (fig. 3) . 

 It is not merely the amount by which the probe is lowered 

 that is subject to expansion, but a considerable length of the 



