324 Mr. A. E. Tutton. An Instrument for [Feb. 14, 



halogen derivatives, whence it follows that for these the ratio of the 

 increase of mean total energy to the increase of kinetic energy of 

 translation of the molecule is proportional to the number of atoms in 

 the molecule. 



IV. " An Instrument for Cutting, Grinding, and Polishing 

 Section-plates and Prisms of Mineral or other Crystals 

 Accurately in the Desired Directions." By A. E. TUTTON, 

 Assoc. R.C.S., Demonstrator of Chemistry at the Royal 

 College of Science, South Kensington. Communicated 

 by Professor JUDD, F.R.S. Received November 28, 1894. 



(Abstract.) 



In a recent communication (' Phil. Trans.,' 1894, Series A, p. 887 ; 

 ' Roy. Soc. Proc.,' vol. 55, p. 108) the author described an instrument 

 for grinding accurately orientated section-plates and prisms of crystals 

 of artificial preparations. The success of that instrument is so com- 

 plete that another instrument has been devised and constructed, 

 npon similar principles, but with the necessary modifications arid 

 additions, to enable equally accurately orientated plates or prisms to 

 be prepared from the relatively harder crystals of natural minerals. 



The new instrument is constructed upon a scale one-fifth larger 

 than the former one as regards such parts as are fundamentally 

 similar, to confer greater strength. The main innovations are those 

 of a cutting apparatus, capable of ready removal, in order not to 

 impede goniometrical, grinding, and polishing operations, and a much 

 larger grinding table fitted with a particularly convenient mode of 

 attachment for any one of nine interchangeable grinding and polish- 

 ing laps, suitable for use with crystals of every degree of hardness. 

 The accompanying illustration represents the lower portion of the 

 instrument, drawn from a point sufficiently high above the base to 

 most clearly exhibit the arrangement of the cutting apparatus. 

 The instrument is not intended to replace the one previously de- 

 scribed, which is fully adapted to all the uses of chemical crystallo- 

 graphers, and the cost of which is only two-thirds that of the one 

 now described. It is intended especially for the use of mineralogists, 

 but, naturally, will likewise serve all the purposes of the smaller 

 instrument. 



The mode of supporting the outer fixed cone within which the 

 movable axes rotate, the construction of the circle and its axis and 

 fine adjustment, and of the gun-metal axis and its counterpoising 

 levers designed for controlling the pressure between crystal and lap, 

 as also of the inner steel axis from which are suspended the crystal a 



