20 



NATURE 



[May 2, 1889 



only been transferred from the steam boilers to the distilling ap- 

 paratus, and that constant breakdowns of the latter necessitate 

 the introduction of sea water into the boiler to eke out the supply 

 of distilled water from the condensers. 



The waters at present in use in marine boilers may be classified 

 as — 



(i) Sea water, (2) distilled water, (3) mixtures of sea water 

 with distilled or fresh water. 



In this paper the nature and causes of the deposits are studied 

 in each of these cases. 



Fresh water contains about twenty to fifty grains per gallon of 

 dissolved solids, principally consisting of calcium carbonate, held 

 in solution by carbonic acid present in the water, whilst sea 

 water contains about 2300 grains per gallon, consisting prin- 

 cipally of sodium chloride, together with magnesium salts and 

 calcium sulphate. The wide difference in composition between 

 fresh and sea water is also shown in the deposits formed by them. 

 Analyses show that with fresh water the incrustation may be 

 looked upon as consisting principally of calcium carbonate ; that 

 with a mixture of fresh and salt water the deposit consists of 

 nearly equal parts of calcium carbonate and calcium sulphate ; 

 whilst the sea water gives, practically calcium sulphate. 



A deposit of calcium carbonate only, separates out as a soft 

 powder, which remains suspended in the water for some time, 

 and can fairly easily be removed from the boiler on cleaning ; 

 whilst calcium sulphate as formed in the boiler separates out in a 

 crystalline form, and binds the deposit into a hard mass, so hard 

 in fact that it requires the aid of a chisel and hammer to detach 

 it from the plates and tubes, an operation which is extremely 

 injurious, and tends to shorten the life of the boiler. 



Calcium sulphate is much more soluble in a saline solution 

 such as sea water than it is in fresh water, but its solubility rapidly 

 decreases (i) on concentration of the saline solution, and (2) with 

 increase of temperature and pressure. 



Sea water having a density of i'027 was evaporated, and 

 analyses made at different densities with the following results: — 



Saline Cous'ituents per Cent, 



Density ... 1-029 1*05 I'og i"22=; 



Sodic chloride 2-6521 4-4201 7*9563 23-8689 



Calcic sulphate ... 0-1305 02175 0-3915 none 



Calcic carbonate ... 0-0103 0-0171 none none 



Magnesic carbonate ... 0-0065 0-0032 none none 



Magnesic chloride ... 0-2320 0-3865 06960 2-0880 



Magnesic sulphate ... 0-1890 0-3150 0-5670 1-7010 



So that on concentrating sea water at ordinary atmospheric 

 pressure, three distinct stages may be traced :— 

 (i) Deposition of basic magnesic carbonate ; 



(2) Deposition of calcic carbonate with remaining traces of 

 the basic magnesic carbonate and hydrate ; and, finally, 



(3) Deposition of the calcic sulphate. 



If the sea water be heated and concentrated 'above a density 

 of 1-225, the salt commences to crystallize out. ' 



These experiments show that if sea water be boiled merely 

 under atmospheric conditions, it would be quite, possible, by 

 taking care that its density does not rise above a certain point 

 (1-09) to prevent the deposition of the calcium sulphate; but 

 any such regulation of density is rendered abortive by the fact 

 that pressure and consequent raising of the boiling-point acts 

 upon the calcium sulphate in solution in exactly the same way 

 as concentration, as it is found that this substance is perfectly 

 insoluble in either sea or fresh water at a temperature of 150° C. 

 In the present high-pressure boilers, even if the sea water be 

 mixed with a hundred times its volume of distilled water, so as 

 to reduce its den.sity very low, deposition of calcium sulphate 

 still occurs. 



Analyses of several specimens of deposits from boilers where 

 sea water was used, showed that in all cases there were two dis- 

 tinct layers— (i) a hard crystalline deposit on the sides of the 

 tubes, consisting of nearly pure calcium sulphate in the form of 

 "anhydrite" ; {2) a softer portion resembling alabaster in the 

 interior, coiisisting of calcium sulphate, with about 6 per cent, 

 of magnesic hydrate. 



The presence of magnesic hydrate in boiler deposits is sup- 

 posed to be due to the mutual decomposition of water and mag- 

 nesic chloride, later experiments have shown, however, that 

 when magnesic chloride and calcium carbonate mutually react 

 upon each other, soluble calcium chloride and magnesium 

 hydrate are formed ; this explains why calcium carbonate is 



never found, except in very small quantities in marine boiler 

 deposits. 



When distilled water only is used, a slight coating is formed, 

 practically consisting only of organic matter, whilst if at any 

 time through a break-down in the distilling app.iratus sea water 

 is mixed with the distilled water, a thin and very hard scale of 

 calcic sulphate is formed. An ^incrustation of this character 

 gave on analysis : — 



Calcic sulphate 90*84 



Magnesic hydrate 0-75 



Sodic chloride 1-41 



Silica 0-85 



Copper carbonate l-ri 



Oxides of iron and alumina 0-24 



Organic matter 2-96 



Moisture 1-84 



This scale is of great interest from the presence in it of the 

 carbonate of copper. It is well known that distilled water has 

 a far greater solvent effect upon metals than a water containing 

 salts in solution, and it is quite conceivable that the distilled 

 water from the surface condensers attacks the brass and copper 

 tubes and fittings, and deposits the copper on the tubes of the 

 boiler, although in only small quantities ; and it is interesting to 

 note that the green spots due to the presence of the copper are 

 all on the under side of the scale — that is, in contact with the 

 metal of the boiler tubes,' showing that in all probability it had 

 been deposited, as suggested, from the water in the boiler, and 

 in contact with the iron would set up local galvanic action and 

 tend to produce pitting. 



The importance of preventing boiler incrustation, and thereby 

 saving the enormous waste of fuel and injury which it entails, 

 has not been without influence on the minds of inventors, and 

 almost every conceivable substance, from potato-parings to com- 

 plex chemical reagents, have from time to time been patented 

 for this purpose, but have failed more or less for marine boilers, 

 because either they have had an injurious effect upon the metal 

 of the plates, or else have produced an enormous bulk of loose 

 deposit, which, although easily cleaned out if the various parts 

 of the boiler were accessible, and if it were only being used 

 intermittently, yet in a marine boiler continuously working, 

 rapidly chokes the portions between the tubes. 



For these reasons, no treatment of the sea water in the boilers 

 themselves is practically possible, and with high-pressure tubular 

 marine boilers the water must be either condenser water, made 

 up to the required bulk with distilled water, as is at present done, 

 or else the condenser water must be augmented by sea water 

 specially prepared for the purpose in a separate apparatus before 

 being supplied to the boilers. 



If the engines of a vessel are in good condition, she will 

 approximately require i ton of water per 1000 horse power per 

 twenty-four hours, to make up the volume of the condenser 

 water to the amount required for the boilers, so that, even sup- 

 posing the engines not to be in good order, and considerable 

 waste to take place, 10 tons per diem would be an outside 

 allowance for even very large vessels. To obtain this amount 

 of treated sea water the author proposes an arrangement, full 

 details and diagram being given in the original paper. 



The sea water, containing 40 pounds of soda crystals to the 

 ton is heated up under pressure in a separate apparatus by passing 

 through the solution superheated steam. Under these conditions 

 the precipitated mixture of calcium and magnesium carbonates 

 becomes very dense and settles very quickly. The water is then 

 forced through filtering frames into the hot well of the boiler 

 and is then ready for use. The whole process is effective and 

 rapid, and simple arrangements are made for flushing out the 

 apparatus, after each batch of water. 



UNIVERSITY AND EDUCATIONAL 

 INTELLIGENCE. 



Cambridge. — The courses of science lectures this term are as 

 numerous as usual, but present few noteworthy features. Mr. 

 Gardiner gives a general course of Botany, while Mr. Darwin 

 is conducting the course of Elementary Biology (Plants), the 

 lectures being given in the Archaeological Museum Lecture 

 Room. Prof. Macalister is lecturing on the History of Human 



