1897.] MICROSCOPICAL JOURNAL. 46 



are followed the English Mechanic tank shall, at any 

 rate, be water-tight. To secure this desirable end, we 

 have to fasten the pillars in their places with cement, and 

 on this cement a great deal depends. It has to be elastic 

 to a certain degree, so as to allow for changes of temper- 

 ature and the consequent expansion and contraction of 

 the glass and wood, it has to firmly resist the passage of 

 the water, and it has to be one that will harden in a fiiirly 

 short time, and that will not smell objectionable, as the 

 living inhabitants of the tank are particularly susceptible 

 to any foulness of the water caused by smells. The two 

 best cements that I know of for the purpose, and which 

 fairly fulfil the conditions required of them, are the fol- 

 lowing : — 



Mix together one pint each of litharge, plaster of Paris, 

 pure white sand, and two-thirds of a pint of freshly 

 powered resin. These are thoroughly incorporated to- 

 gether by turning them over aud rubbing them into one 

 another with the hands, and the mixture is then made 

 into paste with boiled oil and a little driers. It should 

 be of sufficient consistency to dry pretty quickly, but not 

 so stiff but that it will get into the holes and corners 

 easily. If properly made this will not take long to dry; 

 but you must leave the tank for a week at least, or more, 

 before you attempt to stock it; and when you do so you 

 must be quite sure, firstly, that the cement is really hard, 

 and, secondly, that the smell has entirely departed. The 

 second cement is made by melting in an iron ladle over 

 a gas flame or lamp tiiree-parts of pitch and one of gutta- 

 percha. When they are throughly melted and incorpor- 

 ated together, apply liquid, and leave to set. This will 

 not take so long as the other to dry, but it must be left 

 till the smell has departed. It is impossible to lay too 

 great stress on this matter. To use the tank too soon is 

 not merely to court defeat, but to positively insure it. 

 If the lead cement is used, it is advisable to cover it with 



