CHEMISTRY. 



We can now compare the formulas of the sul- 

 phates and nitrates with those of the chlorides : 



Hydric Sulphate, H2,SO 4 ; Hydric Nitrate, H,NO 3 ; Hydric 



Chloride, H.C1. 

 Sodic Sulphate, Na2,SO 4 ; Sodic Nitrate, Na.NOs; Sodic 



Chloride, Na.Cl. 

 Cupric Sulphate, Cu,SO 4 ; Cupric Nitrate, Cu,(NO3) a ; Cupnc 



Chloride, Cu.Clj. 



This comparison shews that NO 3 is equivalent 

 to Cl, while SO 4 is equivalent to Cl a , just as K is 

 equivalent to H, and Cu equivalent to H 2 , or that 

 there is a difference in the value of the symbols 

 of the salt-radicals in our system of notation, similar 

 to the difference in the value of the symbols of 

 the metals. We shall see by-and-by why the 

 atomic weights have been fixed so as to give rise 

 to these differences; our object at present is to 

 explain the use of the system as it is, not to give 

 reasons why it has been made so. It is also obvious 

 that the quantities of the salts above mentioned 

 represented by the last formulae are not equiv- 

 alent to one another ; the equivalent quantities 

 being H.,SO 4 ; Na 2 SO 4 ; Cu,SO 4 ; 2[H,NO 3 ] ; 

 2[Na,N0 3 ]; Cu,(NO 3 ) 2 ; 2[H,C1] ; 2 [Na,Cl]; 

 Cu,Cl 2 . 



It is of the utmost importance that symbols or 

 formulas should always be used as representing 

 definite quantities of the substances, and that the 

 slovenly habit should be avoided of using them as 

 contractions or synonyms for the names of the 

 substances. When we use the words 'water,' 

 ' common salt,' or ' sulphate of potash,' we do not 

 indicate any particular quantities of these bodies ; 

 but when we write H 2 O, NaCl, or K a SO 4 , we mean 

 18 parts of water, 58-5 parts of common salt, or 

 174 parts of sulphate of potash. Just as the 

 quantity of an element represented by its symbol 

 is called its atomic weight, so the quantity of a 

 substance represented by its formula is called its 

 molecular weight. 



Acids such as hydrochloric and nitric, which 

 contain in one molecule one atom of hydrogen 

 capable of being replaced by metal, are called 

 ' monobasic acids ; ' those that, like sulphuric 

 acid, contain in one molecule two such hydrogen 

 atoms, are called ' dibasic acids.' There are also 

 ' tribasic ' and ' tetrabasic ' acids, containing re- 

 spectively three and four atoms of replaceable 

 hydrogen. All acids containing in one molecule 

 more than one atom of replaceable hydrogen are 

 called 'polybasic;' and this polybasic character 

 explains the occurrence of acid and double salts 

 of such acids. Thus, in bisulphate of potash, 

 K 2 O,SO 3 , H 2 O,SO3, or HKSO 4 , we have one of 

 the hydrogen atoms of sulphuric acid, H 2 SO 4 , 

 replaced by potassium. Again, common phos- 

 phoric acid (p. 326) has the formula 3H 2 O,P 2 O 6 , or 

 H 3 PO 4 , and is tribasic, so that we have such salts as 

 Ag s P0 4 , Na 2 HP0 4 , NaH 2 P0 4 , NaNH 4 HPO 4 , &c. 



We can use formulae not only to express the 

 quantitative composition of substances, but also 

 the action of substances upon each other, or gener- 

 ally, chemical changes. We do this by means 

 of what are called ' chemical equations.' Let us 

 take as an instance the action of sulphate of potash 

 upon chloride of barium. Here we have 174 parts 

 of sulphate of potash and 208 parts of chloride of 

 barium before the change, and after it we have 

 233 parts of sulphate of baryta and 149 parts of 

 chloride of potassium. We represent the change 

 by the following equation : 



K,S0 4 + BaCI, = BaSO 4 + 2KC1. 



The formulas representing the quantities of the 

 substances before the change are written first, con- 

 nected by the sign + (which here is the same as- 

 'and") ; then we write = which here stands for 'be- 

 come' or 'are changed into;' and lastly, the formulae- 

 representing the quantities of the substances 

 produced by the change, also connected by the 

 sign +. 



The above equation may then be read as fol- 

 lows : 174 parts of sulphate of potash and 208 

 parts of chloride of barium are changed into 233 

 parts of sulphate of baryta and 149 parts of 

 chloride of potassium. We shall have frequent 

 occasion to use such equations farther on in this- 

 paper. 



The system of atomic or symbolic notation, 

 which we have just explained, originated in the 

 Atomic Theory of Dalton. According to this 

 theory, matter is composed of exceedingly minute 

 ultimate particles or atoms which are incapable of 

 being divided. All the atoms of a given element 

 are precisely similar to one another, but the atoms 

 of one element differ not only in properties, but 

 also in weight, from those of the others ; and 

 although we cannot discover the actual weight of 

 an individual atom, the theory assumes that we 

 can discover the proportion existing between the 

 weights of the atoms of different elements, and 

 that these proportions are represented by the 

 atomic weights. Thus, we do not know the weight 

 of an atom of iron, but, if this assumption is 

 correct, it is four times the weight of an atom of 

 nitrogen that is, in the proportion 56 : 14, In 

 the same way, an atom of mercury is two hundred 

 times as heavy as an atom of hydrogen ; and so- 

 on. 



The theory further supposes that when combin- 

 ation takes place, the atoms of the constituents 

 go together to form groups or molecules ; thus, 

 when copper is heated in oxygen, each atom of 

 copper attaches itself to one atom of oxygen to- 

 form a molecule of cupric oxide. The formula of 

 a compound may thus be considered as a list of 

 the number and kind of the atoms forming the 

 molecule of the compound. 



So far we have considered the proportions by 

 weight in which constituents occur in compounds ;. 

 we shall now look for a little at another way of 

 measuring them namely, by volume or bulk. We 

 can measure the volume or bulk of a quantity of 

 matter, whether it be solid, liquid, or gaseous, and 

 express it by means of units ; and just as we used 

 the word ' part ' to indicate a unit of mass (which 

 may be a grain, or a pound, or a ton), so now we 

 use the word 'volume' to indicate a unit of bulk 

 (which may be a cubic inch, or a cubic foot, or a 

 gallon), only reminding the reader that here, as 

 before, having once fixed which of these is to be 

 our unit, we must adhere to it throughout. 



Chemists have made some progress towards 

 finding out the laws of combination according to 

 volume in the cases of solids and liquids ; but this 

 part of the subject is at present both too compli- 

 cated and too speculative to be profitably dis- 

 cussed here. We shall therefore confine our- 

 selves to the laws of combination of gases accord- 

 ing to volume. The first step towards the dis- 

 covery of these laws was made by Gay-Lussac, 

 soon after the publication of Dalton's Atomic 

 Theory ; and it is a remarkable fact in the history 

 of chemistry that Dalton, who had thrown so 



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