CHAMBERS'S INFORMATION FOR THE PEOPLE. 



salts) ; the atomic weight of mercury is equal to 

 the equivalent of mercury in the mercurous com- 

 pounds (black oxide of mercury and its salts) ; the 

 atomic weight of silver is equal to the equivalent 

 of silver in all its compounds except the peroxide, 

 .and the suboxide and the few salts derived from 

 it, &c. The atomic weight of iron is twice the 

 equivalent of iron in the ferrous salts, and three 

 times the equivalent of iron in the ferric salts. 

 In all cases, however, the atomic weight stands 

 in a simple arithmetical relation to each of the 

 equivalents. 



The way in which this notation is used will be 

 .most easily explained by a few examples. Com- 

 pound substances are represented by formulae 

 expressing the proportions by weight of their 

 elementary constituents thus, HC1 means the 

 compound of I part by weight* of hydrogen 

 and 35-5 parts by weight of chlorine that is, 

 36-5 parts of hydrochloric acid. Here the pro- 

 portion in which the two elements combine is that 

 of their atomic weights, and the symbols are 

 simply placed one after the other. Again, H 2 O 

 means the compound of 2 parts by weight of 

 hydrogen and 16 parts by weight of oxygen that 

 is, 18 parts of water. Here H 2 represents twice the 

 -quantity of hydrogen represented by H, so that 

 ;as H stands for i part of hydrogen, H 2 stands 

 for 2 parts of hydrogen. Again, sulphurous 

 -acid gas contains equal weights of sulphur and 

 -oxygen, and is represented by the formula SO& by 

 which we mean the compound of 32 parts of 

 sulphur and twice 16 parts of oxygen that is, 

 64 parts of sulphurous acid gas. In the same 

 way, SO 3 means the compound of 32 parts of 

 sulphur and three times 16 parts of oxygen, 

 .and this is 80 parts of anhydrous sulphuric acid. 

 CuO means the compound of 63-5 parts of copper 

 .and 16 parts of oxygen that is, 79-5 parts of 

 black oxide of copper, or cupric oxide. Now, 

 1 8 parts of water unite with 80 parts of anhy- 

 drous sulphuric acid to form 98 parts of hydrated 

 sulphuric acid (hydric sulphate). Similarly, 79-5 

 parts of black oxide of copper unite with 80 parts 

 of anhydrous sulphuric acid to form 159-5 parts 

 of cupric sulphate. These compounds may there- 

 fore be represented by the complex formulae 

 H 2 O,SO 3 and CuO,SO 3 respectively ; the comma 

 placed between the formulae indicating that the 

 compounds may be regarded as composed, the 

 one of water and anhydrous sulphuric acid, the 

 other of cupric oxide and anhydrous sulphuric 

 acid. Further, if we place cupric sulphate, which 

 is a white powder, in water, 159-5 P ar ts of it at 

 once unite with 18 parts of water, forming a blue 

 powder, which may be represented by the formula 

 CuO,SO 3 ,H 2 O. This blue powder dissolves readily 

 in water, forming a blue solution ; and if we 

 evaporate the water carefully, we obtain blue 

 crystals, containing, in addition to the water 

 previously in the blue powder, four times 18 parts 

 of water the crystalline substance (crystallised 

 cupric sulphate, or blue vitriol) has therefore 

 the formula CuO,SO 3 ,H 2 O,4H a O. Here 4H 2 O 

 means four times 1 8 parts of water (a large figure 

 placed before a formula multiplying the whole 



* Instead of ' part by weight ' or 'part,' we may here and through- 

 out read 'grain,' or 'ounce,' or 'pound,' or any other unit we 

 please ; we must, however, remember that, having once adopted a 

 .particular unit, we must adhere to it through all the formulae which 

 we wish to compare together. 

 316 



formula up to the first comma). The water 

 present in this salt occurs in two different forms 

 of combination i. Water of crystallisation, easily 

 driven off by heat ; of this there are four mole- 

 cules. 2. Water retained with great force, re- 

 quiring a high temperature to expel it, and called 

 ' water of halhydration ; ' of this there is one 

 molecule. In the same way, hydrated nitric 

 acid, nitrate of potash, and nitrate of copper 

 are represented by the formulae H 2 O,N 2 O 6 ; 

 K2O,N 2 O 5 ; CuO,N 2 O 6 respectively, and crys- 

 tallised nitrate of copper by CuO,N 2 O 5 ,3H 2 O. 

 Again, I part of hydrogen unites with 35-5 parts 

 of chlorine to form 36-5 parts of hydrochloric 

 acid (or hydric chloride), and this is represented 

 by the formula HC1 ; and similarly, chloride of 

 sodium (sodic chloride, common salt) and cupric 

 chloride have the formulae NaCl and CuClj 

 respectively. Looking over these formulae, 



Hydrated Sulphuric Acid 

 Sulphate of Soda 

 Sulphate of Copper 

 HydratedtNitnc Acid 

 Nitrate of Potash 

 Nitrate of Copper 

 Hydrochloric Acid 

 Chloride of Sodium 

 Chloride of Copper 



or Hydric Sulphate HjO.SOj, 



Sodic Sulphate NaaO.SOj, 



Cupric Sulphate CuO,SO3, 



Hydric Nitrate H s O,N 2 Oj, 



Potassic Nitrate K 2 O,N 2 O S , 



Cupric Nitrate CuO, 



i, Hydric Chloride HC1, 



it Sodic Chloride NaCl, 



i Cupric Chloride CuCli, 



we at once see that 2 parts of hydrogen (H^, twice 

 23 that is, 46 parts of sodium (Na 2 ), 63-5 parts 

 of copper (Cu), and twice 39 that is, 78 parts of 

 potassium (Kg), are equivalent to one another 

 that is, are capable of playing the same part in a 

 compound. 



The reader will remember that salts can be 

 represented not only, as has been done above in 

 the case of the sulphates and nitrates, as com- 

 pounds of base and acid (anhydrous), but also as 

 compounds of metal and salt-radical, the salt- 

 radical consisting of the anhydrous acid and the 

 oxygen of the base. A glance at the formulae of 

 the sulphates and nitrates given in the preceding 

 table will shew how this view can be represented 

 by formulae. Thus : 



Hydric Sulphate ................ H 2 O,SOs or 



Sodic Sulphate .................. Na 2 O,SO 3 it 



Cupric Sulphate ................. CuO,SO 3 Cu,SO4. 



Here SO 4 represents the salt-radical of the sul- 

 phates that is, the group of elements which is 

 united with a metal to form a sulphate. This, like 

 many other radicals, cannot be (or at least has not 

 as yet been) obtained as a separate substance, but 

 may still be spoken of and reasoned about as that 

 which is common to all the sulphates, that group 

 which remains unchanged when we pass from one 

 sulphate to another. Similarly, we may represent 

 the nitrates thus : 



Hydric Nitrate ................ ^O.NjOs or H 2 ,N 2 O6. 



Potassic Nitrate ............... K 2 O,N 2 O 5 a K2,N 2 Og. 



Cupric Nitrate ................. CuO.NaOj Cu,N 2 C>6. 



Here N ? O 8 is the salt-radical of the nitrates. 

 But it will be at once observed that N 2 O 6 can be 

 divided by two, or that N S O 6 is the same as 2NO 3 ; 

 we can therefore simplify the above formulae thus : 

 Hydric Nitrate, H,NO 3 ; Potassic Nitrate, K,NO 3 ; 

 and Cupric Nitrate, Cu,2NO 3 or Cu,(NO 3 ) 2 (the 

 group NO 3 between the brackets being multiplied 

 as a whole by the small figure placed after it ; in 

 other words, a group of symbols placed within 

 brackets is treated as a single symbol). 



