1884.] 



MICROSCOPICAL JOURNAL. 



189 



An Iiiiniersion-Appjiratiis for the 

 Deterniiiiatioii of tlie Tempera- 

 ture of the Critical Point in the 

 Fluid Cavities of Minerals. * 



BY ALEXIS A. JULIEN. 



In a former paper, read before the 

 American Chem'cal Society, and pub- 

 lished in vol. iii. of its Journal,! have 

 discussed the subject of the examina- 

 tion of carbon dioxide in the fluid 

 cavities of the white topaz of Minas 

 Geraes, Brazil, and took occasion to 

 sum up the known occurrences of 

 carbon dioxide in fluid enclosures of 

 the rocks and veinstones of this coun- 

 try, so far as detected up to that time. 

 The substance has been unfamiliar 

 even to many chemists, and its occur- 

 rence in nature w^as considered rare, 

 even by most geologists ; but nineteen 

 localities on this continent were then 

 referred to, as determined by Messrs. 

 G. W. Hawes, F. Zirkel, A. Wich- 

 mann, R. D. Ii-ving, A. W. Wright, 

 and, in four localities, by myself. 



Reference was made to the various 

 forms of stage-heating apparatus em- 

 ployed by obsei-vers to produce in one 

 of these minute cavities of a mineral 

 while under obser\'ation through the 

 microscope, the rapid and extensive 

 dilatation of the enclosed floating 

 bubble or sudden vaporization of the 

 liquid, by which the identification of 

 carbon dioxide is easily effected, and 

 to determine accurately the tempera- 

 ture at which this action takes place. 

 This temperature may be very low, 

 such as the radiation of a burning 

 cigar, or of a warm metal rod, a jet 

 of hot air, or of the warm breath con- 

 veyed through a rubber tube, or even 

 of the tip of one's finger (37'c.) ap- 

 plied to the bottom of a thin slide, 

 while resting upon the stage ^ of the 

 microscope, if the mineral section 

 is also very thin. But the exact de- 

 termination of the temperature at 

 which the bubble or liquid disappears 

 requires a special apparatus, of which 

 several forms have been devised, 



•Abstract. Read before Section G, A. A. A. S. 



which may be thus classed according 

 to the thermal agency employed : — 



1. A continuous current of hot air 

 conveyed horizontally through a metal 

 chimney beneath the thin section and 

 the adjacent bulb of a thermometer ; 

 as in the several devices of Nachet, 

 Beale, and JFuess. 



2. A similar current of hot water 

 conveyed between the stage and the 

 slide and thermometer, through a 

 metal conduit from a small adjacent 

 tank or boiler ; as in the apparatus of 

 Polallion, Ranvier, etc. 



3. A conveyance of heat to the 

 thin section and thermometer by con- 

 duction through a metal plate, prefer- 

 ably brass or copper, projecting be- 

 yond the stage over the flames of 

 tapers ; as arranged in the forms pro- 

 posed by Chevalier, Dujardin, Ran- 

 som, and Schultze. (SeeFrey ' The 

 Microscope.') 



4. The conduction of heat derived 

 from a galvanic current under the 

 control of a rheostat, through a fine 

 platinum wire bent back and forth 

 over a ring-shaped thermometer bulb, 

 upon which the slide is supported. 

 This is the apparatus proposed by 

 Vogelsang, fully described in most 

 works on general lithology, as those 

 of Zirkel, Rosenbusch, and Lasaulx, 

 and even yet in common use. 



The objection to all these forms of 

 apparatus lies in their irregular ap- 

 plication of heat, and its irregular and 

 indefinite loss from currents in the sur- 

 rounding atmosphere, and from the re- 

 frigerating eflbct of the mass of metal in 

 the stage, and also in the microscope 

 objective, in an amount proportionate 

 to its close approximation, /. 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 dis- 

 tance to a No. 9 of 0.1 mm. 



Vogelsang suggested the reduction 

 of observations made by means of 

 high-power objectives to the standard 

 of the No. 4, and even was forced to 



