150 



but in most instances without success. The chief cause of failure 

 in these substances is, that, when the water or other menstruum ne- 

 cessary to dissolve them has evaporated, the material will occupy 

 less than its original bulk (in some cases less than one-half). The 

 evaporation does not take place uniformly, but acts partially, begin- 

 ning at the edges ; the dry portion is very hard and unyielding, and 

 the shrinking of the remainder of the substance will either fracture 

 or bend the glass cover, or cause vacuities to appear around the ob- 

 ject, and will eventually crack in every direction. From the very 

 nature of the substance required, water must form a portion of it, 

 which, when evaporated, will cause the material to occupy less space. 

 The remedy for this would be, a medium possessing such great and 

 permanent elasticity as would neither allow vacuities to appear nor 

 bubbles to increase in size, and which in drying should admit of no 

 portion becoming hard and unyielding, but as the moisture evapo- 

 rates, allow the cover to settle down uniformly throughout. 



My attention was first called to a material possessing this quality 

 by Mr. F. H. Wenham. It consisted of a mixture of French gela- 

 tine and a transparent kind of treacle, sold by the sugar-refiners, 

 under the name of " green syrup." This compound, when dry, 

 formed a very elastic material (similar to that of treacle and glue, of 

 which printers' inking-rollers are composed), and several objects 

 have been mounted in it with perfect success ; but the colour of this 

 medium was so perceptible as somewhat to limit its utility. 



From this I was led to discover a compound possessing the elas- 

 tic properties of Mr. Wenham's medium combined with perfect trans- 

 parency. It is composed of the following ingredients : — 



1 oz. gelatine 

 5 oz. water 



5 oz. honey 



\ oz. rectified spirits of wine 



6 drops kreosote. 



Soak the gelatine in the water until soft, raise the honey to boiling 

 heat in another vessel, add it to the moist gelatine, and make the 

 whole boiling hot ; when it has somewhat cooled, but still perfectly 

 fluid, add the kreosote and spirit, previously mixed together ; lastly, 

 filter through fine flannel, or, what is far better, coarse filtering- 

 paper, until perfectly bright. When cold the composition is in the 

 form of a very stiff jelly, which on being slightly warmed runs per- 

 fectly fluid. 



