8 PRINCIPLES OF CHEMISTRY 



can easily be proved that this increase in weight and formation of 

 earthy substances from the metal is accomplished at the expense of 

 the atmosphere, and mainly, as Lavoisier proved, at the expense of 

 that portion which is called oxygen, and, as will afterwards be 

 explained, supports combustion. In fact, in a vacuum, or in gases 

 which do not contain oxygen, for instance, in hydrogen or nitrogen, 

 the iron neither rusts nor becomes coated with scoria. Had the iron 

 not been weighed, the participation of the oxygen of the atmosphere in 

 its transformation into an earthy substance might have easily passed 

 unnoticed, as was formerly the case, when phenomena like the 

 above were, for this reason, misunderstood. It is evident from the 

 law of the indestructibility of matter that as the iron increases in 

 weight in its conversion into rust, the latter must be a more complex 

 substance than the iron itself, and its formation is due to a reaction of 

 combination. Were not this chemical change studied in regard to 

 mass, and did we not know of the ponderability of air, and of its 

 capacity to take part in the phenomena of combustion, we might form 

 an entirely wrong opinion about it, and might, for instance, consider 

 rust to be a simpler substance than iron, and explain the formation of 

 rust as the removal of something from the iron. Such, indeed, was 

 the general opinion prior to Lavoisier, when it was held that iron con- 

 tained a certain unknown substance called * phlogiston,' and that rust 

 was iron deprived of this supposed substance. 



2. Copper carbonate (in the form of a powder, or as the well-known 

 green mineral called ' malachite,' which is used for making ornaments, 

 or as an ore for the extraction of copper) changes into a black sub- 

 stance called 'copper oxide' when heated to redness. 14 This black 



sheet of paper) so as to form a beard about the poles. The balance pan should be exactly 

 under the filings on the magnet, in order that any which might fall from it should not 

 alter the weight. The filings, having been weighed, are set light to by applying the flame 

 of a candle; they easily take fire, and go on burning by themselves, forming rust. 

 When the combustion is ended, it will be clear that the iron has increased in weight ; 

 from 5 parts by weight of iron filings taken, there are obtained, by complete com- 

 bustion, 7 parts by weight of rust. Consequently, if about 5 grams of filings be 

 applied to the magnet, the increase in weight will be clearly seen by the weights that are 

 required to restore equilibrium. This experiment proceeds so easily and quickly that it 

 may be conveniently demonstrated, as a proof of the increase of weight at the expense of 

 air and of its transformation into the solid iron-rust. 



14 For the purpose of experiment, it is most convenient to take copper carbonate, pre- 

 pared by the experimenter himself, by adding a solution of sodium carbonate to a solution 

 of copper sulphate. The precipitate (deposit) so formed is collected on a filter, washed, 

 and dried. The decomposition of copper carbonate into copper oxide is effected by so 

 moderate a heat that it may be accompished in a glass vessel heated by a lamp. For 

 this purpose a thin glass tube, closed at one end, and called a ' test tube," may be em- 

 ployed, or else a vessel called a ' retort.' The experiment is carried on, as described in the 

 third example above, by collecting the carbonic anhydride over a water bath, as will be 

 afterwards explained. 



