136 Laboratory Arts 



for the production of a. reasonably successful result. However, 

 after a few experiments, work such as this ought to be within 

 the power of every laboratory attendant. 



VII. CARE OF BENCH TOPS. 



Bench tops are usually made of teak, and it has already 

 been stated that a fortnightly treatment with a mixture of equal 

 volumes of raw linseed oil and turpentine, well rubbed in, and 

 finally rubbed dry will keep all such tops in excellent condition 

 without danger of causing stickiness. The oil must be raw. 

 Boiled oil will produce a skin on the bench which is a source 

 of endless trouble, and can only be removed as it must be 

 by scraping. 



Bench tops were made, years ago, of cheap wood pine or 

 deal and paraffin wax was melted in with hot irons. Benches 

 such as these are impervious to the action of water, and to 

 some extent of acids, though attacked by strong acids and 

 by alkalies. Little is gained therefore by this treatment, 

 which is difficult to renew, difficult to preserve or improve 

 when stained. It also has a tendency to become incon- 

 veniently highly polished, and to " sweat " under the influence 

 of reflected heat from a sandbath, for example. 



VIII. USEFUL CEMENTS. 



Cements are used for attaching one surface to another. 

 The objects to be cemented may be similar or dissimilar, 

 and may be rough or smooth ; consequently they will need 

 different treatment. 



The action of a cement is to hold together various sub- 

 stances in such position relative to each other as to permit 

 mutual attraction. The joint is always much stronger than the 

 cement, though it could only be of the same strength as that of 

 the cement if a complete layer of cement separated (or joined) 

 the surfaces. The obvious moral is that small quantities of 

 cement only are to be used in every case, and that the surfaces 

 to be joined must be pressed tightly together in the presence 

 of the cement, in order to reduce the quantity to a minimum. 



