PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 543 



change by absorbing more oxygen, and becoming converted into oxide of 

 lead. As already stated, this effect can be produced very quickly by the 

 aid of heat, but equally certain and sure is the action, though requiring a 

 longer time, when exposed to atmospheric influence. 



Tin was well known in very ancient times, and we have reason for think- 

 ing that it was obtained by the Plienecians from Cornwall, in England; 

 1,000 years before Christ. The great contrast between lead and tin com" 

 mences with the ores, as the oxide of tin, or tin-stone, which is the most 

 cbmmon ore from which tin is obtained, is as hard to decompose as lead is 

 easy. 



Tin has, when, pure, almost a silvery whiteness, and will maintain its 

 lustre when exposed to the action of the air for a long time, because it is 

 by no means easily oxydised like lead. 



Nitric acid will not dissolve it, but converts it into an insoluble binoxide 

 of tin. Sulphuric and muriatic acids will dissolve it however, but neutral 

 sulphates and chlorides will not. 



Having thus slightly touched upon some of the most important and cha- 

 racteristic properties of these two metals, we will next consider the action 

 of water upon lead. 



It is frequently remarked, and repeated so often, that it passes as an 

 established fact, that pure water has more action on metallic lead than im- 

 pure water. Your committee incline" to the opinion that this error has 

 caused much misunderstanding, and it is only under certain circumstances 

 that it is true; an explanation of this may serve to make the subject more 

 easily understood. 



The facts of the case appear to be as follows: 



Pure distilled water, from which the air has been expelled by boiling, 

 will not, if excluded carefully from the action of the air, be acted upon by 

 lead; in other words, pure lead will not decompose pure water, if the air 

 is excluded. This is shown by lead remaining bright, when immersed in 

 water under such circumstances But if pure water, either distilled water 

 or rain water containing air, and exposed to the action of the atmosphere, 

 we find a more rapid action upon lead than if saline impurities, like chlo- 

 rides, sulphates and carbonates of soda, lime, &c., were present in small 

 quantities. 



If a moment's reflection is given to this, the reason is evident. When 

 the water is perfectly pure, in order to oxydise the lead, the water must be 

 decomposed and hydrogen set free, while oxygen unites with the lead, to 

 form an oxide or sub-oxide of lead. Taking into consideration the well 

 known difficulty of decomposing water, this looks upon its face unreasona- 

 ble; but when we find the result as stated, we have only a confirmation of 

 our theory. We have, therefore, to look to some other causes than the 

 purity of water, for the action of distilled water upon lead when exposed to 

 atmospheric air, and this we find in the oxygen held in solution and con- 

 stantly absorbed by water from the atmosphere, and it is very easy to ac- 

 count for the change of the oxide of 4ead into carbonates of lead, from the 

 same quarter, as it is well known our atmosphere contains always more or 

 less carbonic acid. 



Having thus traced the action of water upon lead, and shown that to 



