164 Laboratory Arts 



commence by trying to pour out a great deal, and as soon as the 

 balsam begins to flow from the bottle, slice off what is required 

 with a glass plate, and return this plate to the bottle, which it will 

 serve to cover. The result is a globule of balsam free from air- 

 bubbles in the centre of the ground glass plate, gradually spreading 

 toward the edges. Remove the burner, and place the clear plate 

 above the globule of balsam, holding it so that one edge touches the 



corresponding edge of the ground 

 glass plate beneath, and allow the 

 balsam to creep out to the edges of 

 FIG. 117. the plate (see Fig. 117). This may 



take a few minutes, but it must not 



be hurried, and on no account endeavour to increase the speed 

 by pressure upon the upper plate, such a procedure resulting 

 inevitably in disaster. When perfectly clear and free from air- 

 bubbles, allow it to cool, remove from the plate by slipping a 

 knife beneath the bottom glass, and scrape the excess of balsam 

 from the edges with a piece of waste cardboard or glass. Having 

 fairly cleaned the slide, complete the cleaning of the edges by 

 means of a rag dipped in benzene, ether, or xylol, and bind the 

 slide immediately. When the edges are bound the slide may be 

 completely cleaned and polished. It is not necessary that a 

 balsam slide should ever positively dry it rarely ceases to be sticky 

 if left unbound, but once bound it may be cleaned up and no 

 stickiness will be apparent afterwards. Canada balsam, if used 

 as bought, is excessively sticky, and will give great trouble in 

 adhering to the fingers, clothes, and anything else with which it 

 may come in contact. It is not so sticky, however, if partly 

 evaporated, being then more of a crisp solid nature, and easily 

 scraped off glass, etc. At the same time some care is necessary 

 in the handling of the material, and it is well to go slowly and 

 methodically to work. 



Masks may be fixed after the slide is finished ; not, as before, 

 between the glasses, but on the outside. 



Slides made as above, but for temporary use only, may be 

 " cleared " with vaseline or olive oil. They may then be cleaned 

 and used again. 



3. Slides made by Tracing on Plain Glass. Ordinary 

 glass may be used in making lantern slides by the use of special 

 opaque inks. Such are " Pearl " ink a kind of Chinese ink, but 

 this requires to be mixed with a little " ox gall" and "Vitro ink," 

 which is white, but opaque, and on the whole is most satisfactory 

 for hurried work. Vitro ink is sold by Messrs. J. J. Griffin & Sons, 



