128 [November, 1841. 



simple bodies. The largest cube representing Uranium is a little 

 more than six inches on a side. Each cube bears on one of its 

 faces the conventional chemical symbol of the body whose atomic 

 weight it represents, usually the first letter of its name, together 

 with the number expressing its atomic weight. Many of the 

 types are coloured either by a coating of the material represented, 

 or by some convenient pigment exhibiting the natural appearance 

 of the body represented. The number of types of each body 

 varies with the complexity of the compounds into which it is capa- 

 ble of entering ; the greatest number being required of those bodies, 

 which constitute organic substances, viz. carbon, hydrogen, oxygen 

 and nitrogen. 



A second part of the apparatus consists of a light frame support- 

 ing three shelves about fourteen inches apart, open both at the 

 front and rear. This frame is four feet long and two and a half feet 

 high. In order to represent chemical combinations and decomposi- 

 tions, the atom-cubes are first placed separately upon the middle 

 shelf, to represent the case of solution or the liquid state. They 

 are then brought together to indicate mixture of solutions. To 

 represent precipitation, those atoms which are in fact precipitated 

 in actual solutions are carried to the lower shelf; and to represent 

 the products converted into gas, those atoms which constitute such 

 products are carried from the middle to the upper shelf. 



The chemist performs the processes first in his jars or retorts, 

 and then arranges the atoms to indicate the real operations which 

 have been taking place before the eyes of his class. 



In exhibiting the combinations of bodies which may assume a 

 gaseous form, Prof. Johnson makes use of a third apparatus, con- 

 sisting of glass cubical boxes of 100 cubic inches each, and of others 

 containing 50 and 16# cubic inches. 



To represent the combinations and decompositions of gases as in 

 experiments in Eudiometry, both the cubical types and glass boxes 

 are brought into requisition. Thus the type of hydrogen (a cube 

 one inch on a side, marked H 1 ,) is placed on the upper shelf, and 

 over it is inverted a cubical glass box, marked 100, and actually 

 containing that number of cubic inches ; an oxygen type (marked 



