156 SECTION PHYSICS. 



whatever. The secret is a very simple one. Unless I wanted to use 

 the charcoal tube it would be sealed off, and there is virtually no loss 

 of matter nor of charcoal, because, if I seal this off, the same charcoal 

 can be used for another tube, and then the matter which is left in 

 the tube is so excessively small that the original matter I put in is 

 virtually recovered. I will show you first that it is a tolerable vacuum, 

 and then I will show you that the charcoal does contain a very large 

 quantity of matter by heating it. I will attach a coil to it, and Mr. 

 Spottiswoode will be the best judge if it is a tolerable vacuum, and you 

 will see by the strice when I heat it, that it contains a very large 

 quantity of matter. The stria are very wide at first, and by this 

 simple method you can study them in all stages of their formation, 

 from an innumerable number there is a gradual widening until they 

 nearly disappear altogether. You see at once it is a tolerable 

 vacuum from the enormous width of the stria ^ and the condition 

 of the negative pole, which is always the best evidence of a good 

 vacuum, being isolated altogether from the other portion. I will 

 now heat the charcoal in a spirit lamp, and you will see that it 

 holds a large quantity of matter. As I heat it gradually, the gas comes 

 out of the charcoal, and I may tell you at once the substance is bromine. 

 I place a little fluid bromine in this tube, then place it in a water bath, 

 the bromine distils over. The charcoal is heated during the process 

 to drive out all gases, and it absorbs the bromine vapour and 

 creates the vacuum. In this way you can produce a very good vacuum 

 without any machine at all. You see at once the alteration in 

 appearance, as the heat acts on the charcoal, and in a few moments the 

 spark will not pass, the tube getting quite full of bromine vapour. 

 If I had a Bunsen flame I could make it give up all the bromine. 

 There is no special care required in this process ; this is cocoa-nut 

 charcoal, but any charcoal will do. The quantity of bromine used 

 was about a cubic centimetre. You can now see quite plainly the 

 colour of the bromine vapour, but on cooling it will be ail again 

 absorbed by the charcoal. 



The PRESIDENT : Our thanks are due to Professor Dewar for his 

 account of this remarkable instance of the simplification of processes 

 which only those who have been engaged in these matters know how 

 to appreciate. We sometimes get a vacuum by air-pumps and so on. 



