ON THE CHARCOAL VACUUM. 155 



The PRESIDENT : I am sure our thanks are due to Professor 

 Andrews for the account he has given us, and for the very interesting 

 and delicate experiments which he has performed. I am sure you will 

 with me regret that we, like himself, are working under such extreme 

 pressure that we were obliged to ask him to make his communication 

 very short. 



Before calling on M. Sarasin-Diodati I will venture to ask Mr. 

 Dewar to give us a short communication on charcoal vacua. We 

 are fortunate in having him here this morning, and his communi- 

 cation may be properly and naturally placed between the two you have 

 already had and the lecture which is to follow. I have no doubt it 

 will come in very well between the two other subjects best, and he will 

 further imitate nature in allowing that state of transition to be one of 

 extreme brevity in point of time. 



Professor DEWAR : Especially since the recent experiments of 

 extraordinary interest of Mr. Crookes on motion in vacua, a method 

 of readily getting a very perfect vacuum has been one of considerable 

 importance ; and my friend, Professor Clerk-Maxwell, has mentioned 

 my name in his report in the department of physics, dealing with a 

 method of effecting this result. All I have to say is that this plan 

 was originally used by Professor Tait and myself, to improve on the 

 mercurial vacuum. Of course, Sprengel's method may be carried to 

 a very great extent, and will produce a very perfect vacuum as com- 

 pared with an ordinary air-pump. The plan we adopted at that time 

 was as follows : Suppose this tube is to be made a very perfect 

 vacuum. Charcoal is placed in the tube, and the whole exhausted 

 by means of a mercurial air-pump at a red heat ; it is then sealed 

 up, and the last traces of gas, the vapour of mercury, sulphuric 

 acid, and other traces of impurity are absorbed by the charcoal, and 

 in a very short time we found that an electric spark could not 

 pass. That has been shown very often at the Royal Institution. The 

 advantage of this method was that by heating the charcoal again you 

 could at once produce stria; it goes backwards and forwards as often 

 as you like. The improvement which I have made on this within the 

 last two or three days, is one which, I daresay, may interest Mr. 

 Spottiswoode, in connexion with his experiments on stricz. This 

 vacuum is made by charcoal without the use of any mercurial pump 



