CHAMBERS'S INFORMATION FOR THE PEOPLE. 



much light on the laws of combination according 

 "to weight, was inclined to doubt the accuracy of 

 -Gay-Lussac's views. These views were afterwards 

 -greatly extended and simplified by Avogadro ; 

 and subsequent investigation and comparison have 

 ^established what we may call 'Avogadro's law' so 

 fully, that it may be said to be as important a 

 corner-stone of the chemical edifice as the Atomic 

 Theory itself. Without entering into historical 

 questions, which are beyond the scope of this 

 paper, we shall state this law, as it is at present 

 held by the great majority of chemists, thus: A 

 volume of any gas contains the same number of 

 -molecules as the same volume of any other gas, 

 the two volumes being measured at the same 

 -temperature and pressure. (See foot-note, p. 

 311.)* It follows at once from this law that the 

 specific gravities of two gases (taken at the same 

 temperature and pressure) are in the same propor- 

 ~tion as their molecular weights ; and we have 

 thus a means of determining the molecular weight 

 of any substance which either is a gas at ordinary 

 temperatures, or can be, without chemical change, 

 converted into one by the action of heat, by com- 

 'paring its specific gravity in the gaseous state 

 with the specific gravity (at the same temperature 

 and pressure) of some gas the molecular weight of 

 -which is known. 



By applying this method, we find that the molec- 

 ular formula of a substance is not always the 

 'formula which most simply represents its compo- 

 sition. 



Thus, let us assume that the formula of hydro- 

 chloric acid gas is HC1, and therefore its molecular 

 -weight 36-5, and from this let us deduce the molec- 

 ular weight of some other gases. The specific 

 -gravity of hydrogen is to the specific gravity of 



Hydrochloric Acid. ...HO. 

 Hydrobromic Acid....HBr. 



Hydriodic Acid HI. 



'Chloride of Sodium ...NaCl. 

 Bromide of Potassium. KBr. 



Water H 2 O. 



Sulphuretted Hydrogen.. HjS. 

 Seleniuretted Hydrogen . . HjSe. 



Chloride of Calcium CaCl 2 . 



Bromide of Zinc. ZnBrz. 



hydrochloric acid almost exactly as 1 : 18-25, or as 

 2 : 36-5 ; therefore, if the molecular weight of 

 hydrochloric acid is 36-5, that of hydrogen must 

 be 2, and its formula Hj. Again, the specific 

 gravity of hydrochloric acid is to that of nitrogen 

 almost exactly as 36-5 : 28 ; therefore, the molec- 

 ular weight of nitrogen is 28, and its formula N. 

 In these cases, then (and the same is true of many 

 other gases), the theory leads us to believe that 

 the atoms are united in pairs, each pair being a 

 molecule, and that, just as in hydrochloric acid 

 each hydrogen atom is united with an atom of 

 chlorine, so in hydrogen gas each atom of 

 hydrogen is united with another atom of hydro- 

 gen. Again, the well-known substance benzol is 

 a compound of carbon and hydrogen in the pro- 

 portion of 12 parts of carbon to i of hydrogen ; 

 the simplest formula by which its composition 

 could be represented is obviously CH. If this were 

 its true formula, the specific gravity of its vapour 

 should stand to that of hydrochloric acid gas (at 

 the same temperature and pressure) as 13:36-5, 

 but we find the proportion almost exactly 78 : 36-5 

 that is, 6 times 13 : 36-5. The molecular weight of 

 benzol is therefore 6 times 13, and its formula 

 C 6 H 6 . In considering individual substances, we 

 shall meet many such cases, and need not, there- 

 fore, dwell longer on the subject here. 



THE DOCTRINE OF ATOMICITY. 



In order to illustrate what is called 'the Doc- 

 trine of Atomicity,' we shall give in a tabular form 

 the molecular formulas of a number of substances, 

 selected so as to place this 'doctrine' before the 

 reader in the way in which it may be most easily 

 understood : 



Marsh Gas CH<. 



Siliciuretted Hydrogen. . SiHx. 

 Perchloride of Tellurium. TeCU. 



Perchloride of Tin RnPl . 



Bromide of Platinum. . 



Ammonia 



Phosphuretted Hydrogen . . PHs. 



Arseniuretted Hydrogen . .AsHs. 



Chloride of Gold AuCls. 



Terbromide of Phosphorus. PBrj 



Looking at these formulae, the reader will easily 

 see that there are some elements, such as hydro- 

 gen, chlorine, bromine, iodine, sodium, and potas- 

 sium, which unite with one another two and two 

 to form compounds containing two atoms in the 

 molecule, one of the one, the other of the other 

 element. Such elements are called 'monads.' 

 Again, there are elements, such as oxygen, sul- 

 .phur, selenium, calcium, and zinc, which unite with 

 monads to form compounds containing three 

 atoms in the molecule, one atom of the element in 

 -question uniting with two monad atoms (as in the 



second column). Such elements are called dyads. 

 Further, we have elements, such as nitrogen, phos- 

 phorus, arsenic, and gold, each atom of which 

 unites with three monad atoms to form a mole- 

 cule. Such elements are called triads. In the 

 same way there are elements, such as carbon, 

 silicon, tin, and platinum, which are called tetrads, 

 each atom uniting with four monad atoms to form a 

 molecule. These different modes of combination 

 can be represented graphically by drawing lines 

 uniting the symbols of the atoms forming the 

 molecule, thus : 



H Cl 



Hydrochloric Acid. 



N& Br 



Bromide of Sodium. 



H O-H 

 Water. 



Cl Ca Q 



Chloride of Calcium. 



H 

 H N H 



Ammonia. 

 Cl 



ci P a 



Terchloride of Phosphorus. 



H 



H C H 

 H 



Marsh Gas. 



Cl 



ci sn a 



C, 



Stannic Chloride. 



In these graphic formula it will be observed that 

 a monad atom is represented by its symbol with 



* This law is in complete accordance with the 'Dynamical 

 "Theory of Gases,' as at present held by physicists. 

 318 



one line proceeding from it ; similarly, two lines 

 proceed from the symbol of a dyad atom ; three, 

 from that of a triad ; and so on. 

 The property of combining with a definite number 



