Prof. Min chin's Experiments in Pliotoelectricity. 219 



glass place a dish, D D, supported by a stand S, which allows 

 the dish to be raised or lowered ; the dish being raised 

 until it nearly touches the glass g, pour into it, until the tin 

 plate p is completely covered, a portion of the following 

 mixture : — 



500 c. c. of distilled water, 

 3 c. c. of pure nitric acid, 

 15 grammes of nitrate of ammonia. 



If the tin is pure and has been thoroughly cleaned, the 

 moment the plate p is covered by the solution, a whitish 

 deposit, is thrown down on its surface. The plate may be 

 left in the solution for 3 or 4 minutes, and then the dish D D 

 is lowered and removed. The deposit on the tin should be 

 uniform and must not be allowed to form for more than 

 4 minutes. When the dish D D has been removed, the 

 under surface of the glass plate g g should be dried by blotting- 

 paper. Then a process of heating must take place. The 

 flame of a spirit-lamp must be applied uniformly to the under 

 surface of the glass plate, i.e. moved backwards and forwards 

 until the whole of the liquid on its upper surface is evapo- 

 rated. Great care must be taken not to melt the tin plate p 



Continuing the heating process, the upper surface of the tin 

 passes through a series of appearances. At first (i. e., when 

 the water has just been evaporated) the deposit has a dull 

 slaty look ; as the heating goes on, this changes to a whitish 

 aspect, a gas with a nitrous smell coining off ; as the heating 

 is continued, this whitish surface undergoes a rapid change 

 to a dark colour with, apparently, a tinge of green, which 

 travels like a shadow from one end of the plate to the other ; 

 the heating being still continued, this dark surface changes 

 to a strong white, and the flame ought to be kept under it 

 until the gas is completely driven off. 



The plate should then be plunged into alcohol. It is now 

 in the most sensitive condition. I have found that if the 

 heating is stopped at the end of the penultimate stage — viz., 

 that in which the dark greenish colour has been reached — the 

 plate will be very fairly sensitive to light. It may be 

 suspended from a platinum wire fixed through a pin-hole at 

 one end of the plate, and when placed in an alcohol cell in 

 front of a clean tin plate similarly suspended, these plates 

 being connected with the poles of an electrometer while the 

 cell is screened from light, a small difference of potential will 

 be observed, the sensitized plate being positive to the unsen- 

 sitized ; but this difference of potential will usually disappear 

 after a short time. 



