ZOOLOGY AND BOTANY, MICROSCOPY. ETC. 1069 



it to obtain extinction of the field. Once for all approximate estima- 

 tions of the temperature of the field might be made in terms of the 

 resistance of the platinum strip, the variation of such resistance with 

 rise of temperature being known. Such observations being made on a 

 suitably protected strip might be compared with the wedge readings, 

 the latter being then used for ready determination. 



The mineral to be experimented on is placed in small fragments 

 near the centre of the platinum ribbon, and closely watched while the 

 current is increased, till the melting-point of the substance is apparent. 

 Up to the present Mr. Joly has only used it comjDaratively, laying 

 fragments of different fusibilities near the specimen. In this way ho 



Fig. 2il. 



Las melted beryl, orthoclase, and quartz. Mr. Joly has been using 

 the apparatus for nearly a month, and in its earliest days it led him 

 right in the diagnosis of a microscopical mineral, iolite, not before 

 found in Irish granite. The unlooked-for characters of the mineral, 

 coupled with the extreme minuteness of the crystals, led him pre- 

 viously astray, until the meldometer fixed its fusibility as far above 

 the suspected bodies. 



A form of the apparatus has been adapted, at Professor Fitzgerald's 

 suggestion, to fit into the lantern for projection on the screen. In this 

 form the heated conductor passes both below and above the specimen, 

 which is regarded from a horizontal direction. 



Mr. Joly writes us : — " The figs, represent the improved form of 

 the meldometer ; in which the clamps of the stage can be used to hold 

 it firm against the drag of the wires connecting it with the battery. 

 The platinum strip is held by two forceps bound to the hearth of the 

 meldometer by the binding screws, taking the leads but free to turn 

 round the shafts of these screws, so that, on rotating the little adjust- 

 ing screw shown at a, the forceps are brought nearer or further 

 apart. The object of this is to take up the sag of the platinum strip, 

 which becomes very considerable at high temperature. The forceps 

 are opened when inserting the platinum by turning the little screws 

 h b. In the figure the jaws of these forceps are shown so shaped as 

 to tend to imjjress a trough or channel form on the strip, which is 

 advantageous both for the purpose of keeping the specimen from 

 falling off and also as further insuring its being at the temperature 

 of the strip." 



Stokes- Watson Electric Spark Apparatus. — Messrs. Watson have 

 modified this apparatus as shown in fig. 242, substituting for tho 

 hingle electrode of the original form * a second disc of six electrodes. 



* See this JouirmI, iv, (1881) p. 'J(M. 



