318 SUMMARY OF CURRENT RESEARCHES RELATING TO 



much of the gas as is allowed to escape from between the top of the mercury 

 and the bottom of the gas delivery tube to the burner e. The regulation 

 of this apparatus so as to obtain a static temperature, as is well known, is a 

 matter of detail depending chiefly on the careful use of the mercurial screw- 

 plug / and the height and intensity of the burner e. A temperature quite 

 as accurate as is needed can be obtained for the purpose required. 



The stage (A) is placed in position on the instrument ; and two 

 openings in this hollow stage at c d (A) are connected with two similar 

 openings in the water-vessel, viz. g h (B). The whole is carefully filled 

 with water and raised to the required temperature and regulated. 



The manner in which it accomplishes the end desired is as follows. On 

 the centre of the stage (A) will be seen a small cylinder of glass: this is 

 ground at the end placed on the stage, and covered with a sort of drum- 

 bead of indiarubber at the upper end. By examining C with a lens it will 

 be seen that a cell is countersunk into the upper plate of the hollow stage 

 at e", and a thin plate of glass is cemented on to this (seen also in section 

 in the same figure). At e another disc of gla,ss is cemented watertight, 

 so that a film of warm water circulates between the upper and under 

 surfaces of this glass aperture. A glass cup is placed in the jacketed 

 receptacle / (A and C), and this also is filled with vrater. A piece 

 of linen is now laid on the stage (A, g), with an aperture cut in its 

 centre slightly less than the countersunk cell in which the glass disc e" is 

 fixed, and a flap from it is allowed to fall over into the glass vessel / 

 (A and C). Thus by capillarity the water is carried constantly over 

 the entire face of the linen. But the glass cylinder seen in A is made of a 

 much larger aperture than the cell and the opening in the linen, and 

 consequently a large annulus of the linen is inclosed within the cylinder. 

 The drop of fluid to be examined is placed on the small circular glass 

 plate and covered with the thinnest glass, the drum-head cylinder is placed 

 in position, the point of a high-power lens is gently forced upon the top 

 of the indiarubber through a small aperture, thus forcing the lower ground 

 surface of the cylinder upon the linen, and making the space within the 

 closed cylinder practically air-tight, but still admitting of capillary action 

 in the linen. Thus the enclosed air becomes saturated. 



By complete circulation the water in the vessel e (A) is but slightly 

 below that within the jacket of the stage, and thus the vapour as well as the 

 stage are near the same thermal point. 



For aiding in illumination and admitting various illuminating apparatus, 

 a large bevelled aperture e (A) is made between the lower and upper 

 plates of the stage jacket which is found to supply all the accommodation 

 needed. 



Merian's Arrangement for Heating Minerals.* — Herr A. Merian, 

 following the researches of Mallard and Klein on the influence of heat on 

 boracit, has studied other mimetic minerals in a similar manner. In 

 tridymit no change of its optical relations could be perceived when the 

 ordinary hot stage was used, and for the purpose of observing its behaviour 

 at still higher temperatures the following arrangement was adopted. 



A Microscope a (fig. 82) was so fixed in a box / that daylight could be 

 made to pass from a plane mirror c through a convex lens to the nicol on 

 the stage h, the Microscope being horizontal. The space between the stage 

 and the objective was sufiiciently large to allow the introduction of the 

 preparation d and the heating apparatus. 



* Neues Jahrb. f. Mimral., Geol., u. Palseontol., 1884, pp. 193-5 (2 figs.). 



