86 THE WOMBAT. 



Any one who has employed the vertical method of copy- 

 ing will appreciate the convenience of always having the copy 

 square with the camera, having no tripod to manoeuvre with, 

 and having good even illumination. Back the plates used for 

 making the negative copies and also the lantern plates, 

 whether used in contact with the negative or by reduction in 

 the camera, and you will be well repaid for your trouble in 

 the improvement in the results obtained — I might even say 

 the " crispness." Any of the usual backing mixtures may be 

 used, the caramel and sienna being as good as any. 



Plates for negatives. For negatives of copies in half 

 tone, as photographs, wash-drawings, etc., ordinary plates of 

 any make are suitable, but for copies of line or screen work a 

 good plan is to use lantern plates for the negative, as perfectly 

 clear glass and ample density may be so easily obtained. 



PORTLAND CEMENT. 



By Richard Taylor. 



Read before the Gordon College Science Club, 21/5/97. 



Although Portland Cement is the subject I intend 

 making a few remarks about, I will, in the first instance, leave 

 the word Portland out, and say a few words about cement. 

 Cement is a substance which is capable of assuming a liquid 

 form, and of being applied between the surfaces of bodies so 

 as to unite them firmly when solidifying, such as glue, paste 

 or shellac, but the cement I wish to speak about is one which 

 has for its chief constituent carbonate of lime; in fact carbonate 

 of lime and cement are so closely allied that it is impossible 

 to speak of one without touching upon the other ; therefore 

 I will first of all direct your attention to carbonate of lime. 



Carbonate of lime occurs as a mineral in all 

 geological ages, and it is very widely and abundantly dis- 

 tributed all over the globe. In its natural state it is largely 

 used for building purposes, and when burnt it becomes quick- 

 lime, which is used for hundreds of different purposes in our 

 arts and manufactures. 



Limestone is divided into many qualities, but for the 

 present purpose I will only make three divisions : — 

 1 st. The fat or rich. 

 2nd. The poor. 

 3rd. The intermediate or hydraulic. 



1. Fat or rich lime is that which is most free from foreign 

 matter ; when water is poured on a piece it hisses, crackles, 



