466 SUMMARY OF CURRENT RESEARCHES RELATING TO 



then screwed up and the gas-jet phxced at the end of the heating tube. 

 As the water gets warm its excess escapes from the tube through which 

 it was introduced. In about one hour to an hour and a half, when the 

 thermometer marks 36° to 38° C, the tube is closed with the caoutchouc 

 plug. As the water gets hotter it mounts in the glass tube and causes a 

 pressure on the caoutchouc membrane of the regulator, and this lowers the 

 flame by diminishing the current of gas supplied. If the temperature 

 lowers the water descends and the gas is supplied more freely. Should 

 the apparatus have been regulated for too high a temperature some water 

 is introduced into the tube by means of a fine pipette, and per contra some 

 is withdrawn by removing the caoutchouc plug if the temperature has 

 been regulated too low. It is stated that the regularity of this hot stage 

 is such that even under unfavourable conditions it does not vary more than 

 a few tenths of a degree. 



Julien's Immersion Heating Apparatus.* — Dr. A. A. Julien's " immer- 

 sion apparatus " was devised for the special purpose of exactly determining 

 the temperature of expansion of the liquid in the fluid cavities of minerals. 

 He considers that most of the forms hitherto devised are " extremely inac- 

 curate, often complex and untrustworthy, and it may be owing to this cause 

 that Brewster obtained, for the critical temperature of the liquids in 

 quartz, results of the very wide range between 20° and 51° C." 



The author in a previous paper thus expressed himself on the subject. 

 " The objection to all these forms of apparatus lies in their irregular appli- 

 cation of heat, and its irregular and indefinite loss from currents in the 

 surrounding atmosphere, and from the refrigerating effect of the mass of 

 metal in the stage, and also in the objective, in an amount proportionate 

 to its close approximation, i. e. to its focal distance or high power. Even 

 in the most pretentious apparatus, that of Vogelsang, its inventor admits 

 a variation or error of 10° C, according to the objective employed ; from a 

 No. 4 Hartnack of 3 mm. focal distance to a No. 9 of • 1 mm. Vogelsang 

 suggested the reduction of observations made by means of high-power 

 objectives to the standard of the No. 4, and was even forced to make a 

 plus correction of 1° C. for observations in which the temperature of the 

 air of the room and of the Microscope fell below^ his normal (20° C.) as far 

 as 12° to 15°. Practically, in use these observations are consequently made 

 almost altogether on large cavities and under low-power objectives, and 

 an accuracy to 1° C. has been accepted as satisfactory. Although wide 

 discrepancies have constantly occurred, even in determinations on the fluid 

 cavities in the same slice of mineral by means of these devices, on the 

 other hand some of the most delicate and important investigations, such as 

 those of Sorby and King on the indication of the degrees of pressure to 

 which certain granites have been subjected during folding and metamor- 

 phism, have rested largely upon the accuracy of determinations of this 

 very kind." f 



Brewster, Sorby, and Hartley have used the same principle as the 

 author. Hartley adopting the plan of immersing the slide in water of known 

 temperature, removing, wiping it hastily, placing it on the stage, and 

 instantly examining it J Far more accurate results with greater conveni- 

 ence can, however, be obtained by means of an apparatus jiermitting the 

 slide to remain under observation, immersed in a layer of water on the 

 stage, and continuously warmed by a current of air from the breath of the 

 observer, or, if necessary, by the conduction of heat to the bottom of the 



* Journ. N. York Micr. Soc, i. (1885) pp. 137-9. See also this Journal, 1882, p. 266. 

 t Aiuer. Mon. Micr. Jouru. v. (1884) pp. 189-90. 

 i Journ. Chem. Soc. London, 1876, p. 139. 



