282 Tlie Philosophical Society of Glasgow. 



both methods has been calculated, — the chemical one of neutralizing 

 the sulphate of lime with soda, and the mechanical one of an abundant 

 discharge and supply, so as to keep the sulphate of lime under its crys- 

 tallizmg quantity. 



It is necessary for this purpose to know the relative proportions of 

 feed-water, and water required to be discharged, in order to prevent 

 scale or crust. Many writers treat this crust as if it were common salt, 

 and instruct how to make and graduate instruments for ascertaining its 

 quantity, the graduations being efifected by observing the depths to 

 which the instrument sinks in water in which certain proportions of 

 common salt has been dissolved. They say, " Sea water contains 3 

 per cent, of salt, and when the boiler contains less than 12 per cent, 

 there will be little or no crust," therefore, it is necessary to blow off' 



3 1 



ths or -jth of the feed-water, in order to prevent the formation of 

 12 4 



crust. This reasoning, however, is unsatisfactory, as it is evident to any 



one who has the sense of taste, that the crust is not common salt ; and 



chemical analysis shows that sea-water from the English Channel, 



although it contains nearly 3 per cent, of common salt, contains only 



about 0.8 per cent, of the materials forming the crust, and only 



0.14 per cent, of the material of which, according to Mr. Napier, 



upwards of 90 per cent, of the crust is composed. It is also 



shown by analysis that a satui'ated solution of this material, sulphate 



of hme, in cold distilled water, is as 1 to 380, and as 1 to 388 in 



boiling water, or 25-7 parts of lime to 10,000 of solution. Mr. Napier, 



however, found 203 grains of sulphate of hme per gallon, in water 



taken from a boiler off Ailsa Craig. Its density is not stated; but I have 



assumed it to contain twice its natural quantity of saline matter, or its 



density to be 1 -0548, sea- water being 1'027-i; this gives the ratio 203 to 



73,836 or 1 of sulphate of lime to 3G1 of solution, or 27--17 of sulphate of 



hme to 10,000 of solution. This proportion, it is inferred, is either 



a saturated solution, or such as the engineer of the vessel found little 



or no crust formed in. For want of better data, 28 of sulphate of lime 



to 10,000 of solution is assumed as the limit of saturation in boilers 



using sea-water, working at pressures not exceeding twenty lbs. above 



14 

 the atmosphere. This is equivalent to discharging ^, or one-half of 



the feed-water. This assumption is confirmed by the practice of 

 the British and North American Mail Company ; by Mr. Napier's Ailsa 

 Craig engineer, who was evidently blowing off nearly this amount ; and 

 by an experiment of Mr. Thomas Rowan, one of Dr. Penny's pujiils, 



