264 TOWERS AND TANKS FOR WATER-WORKS. 



of greater density, as the resin dissolved in the oil and asphalt 

 tends to make a more compact substance, and partly because 

 it increases its electric insulating power, also in consider- 

 able measure because such a resin is very indifferent to the 

 action of sulphur-gases. For all these reasons it seems to the 

 writer that the maximum of durability is only to be reached 

 by a compound of hard asphaltum, copal-gum, and linseed- 

 oil, thinned, if necessary, with pure turpentine. It is of the 

 highest importance that the oil employed should be so refined 

 as to have its non-drying constituents removed, so as to avoid 

 as much as possible the use of dryers. This is of more im- 

 portance than in a pigment and oil-paint, because the most 

 obvious thing about asphalt is mentioned in the observations 

 of M. Riffault, made some thirty or forty years ago, that 

 ' asphalt destroys the drying quality of oil.' 



This is due to the fact that, being a viscous substance, it 

 closes the pores of the oil and thus obstructs the entrance of 

 air and moisture, which is also the cause of the great dura- 

 bility of such compounds. 



Not only is it necessary to have the most suitable materi- 

 als in such proportions as experience has shown to be best, 

 but the ingredients should be compounded in the most 

 approved manner. 



Long experience has shown that there are certain tempera- 

 ture-curves to be followed in combining certain materials, 

 differing for different compounds, a departure from which in- 

 jures the durability of the resultant compound. The upper parts 

 of the curves approach dangerously near to the decomposing 

 point of the oil, and it has been found that a suitably refined 

 pure oil has that point more than IOO deg. F. higher than 

 common oil; it is on this account, also, important to use the 

 highest skill in the manufacture. The choice of ingredients 

 is of less importance than their proper proportion, and this 

 again is of no more value than the use of the best process 



