732 THE MICROSCOPE. 



the uninitiated for the result of their action on the siih- 

 stances under examination. 



Dr. Guy's^ method of procedure is as follows : — " Pro- 

 vide small crucibles, covers, slabs, or fragments of white 

 porcelain ; a few microscopic cell-glasses, with a thickness 

 of about one-eighth of an inch and a diameter of circle of 

 about two-thirds of an inch : and discs of window-glass 

 about the size of a shilling. Place the porcelain slab on 

 the ring of a retort-holder or other convenient support ; 

 then the glass cell ; and upon the porcelain in the centre 

 of the cell a minute portion of the alkaloid or other white 

 powder, or crystal reduced to powder ; then pass the clean 

 glass dish through the flame of the spirit-lamp till the 

 moisture is driven oif, and adjust it with the forceps over 

 the glass ring ; now apply the flame of the spirit-lamp to 

 the porcelain, underneath the powder or crystal, and con- 

 tinue the heat till the powder undergoes its characteristic 

 change and gives off vapour. Watch the deposit of this 

 vapour on the glass dish, and remove the spirit-lamp, 

 either directly or after a short interval, as experience may 

 determine. 



" The white surface of porcelain being visible through 

 the glass disc, as through a window, the behaviour of 

 the substance under examination is easy to observe. It 

 may be driven ofl" without undergoing change or leaving 

 residue, and the disc may be covered with crystals, as 

 happens with arsenious acid, or with an amorphous 

 sublimate, as happens with calomel ; it may coalesce, 

 throw out long silky crystals, to be gradually transferred 

 as crystals to the glass disc, as is the case with corrosive 

 sublimate ; and it may melt, with or without previous 

 change of colour, retain or shift its place, deposit carbon 

 more or less abundantly, and yield a sublimate of detached 

 crystals (veratrine), twigs (solanine), tufts (meconine), 

 branching patterns (strychnine, morphine, cryptopia, &c.), 

 watered patterns with or without crystalloids (several alka- 

 loids and glucosides), the melting and deposition of carbon 

 being a common property of the alkaloids and of some 

 analogous active principles. 



(1) Dr. W. A. Guy, F.R.S. (fcc on the "Sublimation of the Alkaloids."— 

 Fhcirmaceutical Joiirncd, June and August, 1867. Micros. Jour. Dec. 1807. 



