354 



♦ KNOWLEDGE ♦ 



[June 15, 1883. 



sun, acting on the ocean's surface, vapourises water only ; 

 tlirougli the cycle of cloud, mist, and rain, it is again 

 borne to earth, and once more couimonces its downward 

 How oceanwards, carrying with it another burden silently 

 stolen from river banks, and as silently leaving its load 

 with the sea as it goes on to repeat its mission in cloud 

 and shower. In this way the sea acquires its characteristic 

 saltness tlirough the' gradual transference, by the agency 

 of rivers, of the soluble matter of land to itself. The 

 saltness of the sea is not the result of the solution of one 

 substance alone ; although by far the greater part of the solid 

 residue left on evaporating a sample of sea-water to dryness 

 consists of salt proper (common salt or sodium chloride, 

 NaCl of the chemist), there are several other ingredients 

 also present. Among these may be mentioned magnesium 

 chloride and bromide, calcium sulphate and carbonate, 

 l)esides traces of other substances. The presence of mag- 

 nesium chloride may be noticed by tasting sea-water ; it 

 will be observed that it has, in addition to the simply 

 saline taste of pure salt, a bitter flavour. This is much 

 iutensitied by e%aporating a quantity of the water almost 

 to dryness ; the salt, being less soluble, crystallises out, 

 and leaves an intensely bitter solution of the more soluble 

 magnesium chloride. To this solution, to which we shall 

 again refer, the name of bittern is given. The quantity of 

 calcium sulphate in sea-water amounts to a little over one 

 part per 1,000, yet, although so small, it is suflicient to 

 produce vast effects in Nature, as when, for instance, through 

 an alteration in the configuration of the land, a portion 

 of the sea is changed into an inland lake and then 

 slowly evaporated; beds of rock-salt are thus formed, 

 iuterstratitied with layers of gypsum, which is but. 

 crystallised calcium sulphate. As the substances dissolved 

 in sea-water must necessarily have come from the land, and 

 as the igneous rocks must have furnished the first material 

 to be weathered and ground down into sediment, it is inte- 

 resting to trace the presence of the same elements in these 

 primary rock masses. Igneous rocks by their decomposition 

 furnish two classes of bodies ; those of one of these are 

 insoluble, and comprise sand, which is mostly pure silica 

 and clay — a silicate of alumina ; among the soluble bodies 

 are compounds of sodium, potassium, magnesium, and cal- 

 cium. These are also present in sea-water. The presence 

 of common salt in granite may be easily proved by 

 a striking experiment. Take a few fragments of the 

 rock and crush them to powder in a clean iron mortar, 

 or else on a clean surface, with a hammer, transfer the 

 powder thus obtained to a test-tube, and add distilled 

 water, boil, and pass the solution through a filter-paper. 

 Evaporate a few drops to dryness on a clean glass slip ; a 

 sensible residue remains. To another portion add a few 

 drops of silver nitrate solution ; the production of a white 

 precipitate of silver chloride proves the presence of a soluble 

 chloride in the rock. Viewed under the microscope, many 

 samples of granite show themselves to be full of minute 

 cavities, in many cases partly filled with a saturated 

 salt solution. Before leaving the historic chemistry 

 of sea-salt, taught us by geology, a \ery remarkable 

 ])roperty of sea-water, as distinguished from fresh water, 

 may be mentioned. Select two glass vessels of similar 

 size and shape (tall gas jars answer admirably), put into 

 each a couple of ounces of dried mud, the finer the better, 

 and till the one with fresh, the other with salt water ; shake 

 them both thoroughly and then set aside. The mud will 

 be deposited as a sediment from the sea-water, leaving the 

 upper liquid clear, in a fraction of the time necessary to 

 i>Hect the same with the fresh water. This is no doubt 

 one reason why many rivers deposit their mud as a delta 

 innnediately at their mouths, rather than carry it out and 



allow it to settle for a muck further distance over the sea- 

 bottom. 



At one time the extraction of salt from sea-water wm 

 carried on largely on the southern shores of England. 

 Although the working of the \ast salt deposits of Cheshire 

 has now, with us, superseded this industry, yet in the 

 south of Europe the process is still solely eniployed. 

 Shallow pools are constructed at high-water mark, the salt- 

 water is allowed to evaporate in them ; and, from time to 

 time, as they form, the thin crust.s of salt are raked off: 

 these constitute the haij-aaH of commerce. After as much 

 salt as possible has thus been obtained, the remaining 

 water, or Ijittern, is employed as a source from which other 

 substances are prepared, the most important of these 

 being bromine. The presence of this peculiar element 

 in sea-water may be proved by the following experiment 

 Pour some three or four ounces of sea-water in a 

 si.x or eight-ounce stoppered bottle, and add a few 

 drops of pure sulphuric acid, and then some of a solu- 

 tion of potassium permanganate ; the superb violet 

 colour of this latter compound disappears almost 

 immediately, giving place to a deep sherry-colour ; at 

 the same time the liquid acquires a most pungent and 

 irritating odour. Notice, too, that the upper part of the 

 bottle is filled with a reddish-tinted vapour. The addition 

 of the potasium permanganate has produced a chemical 

 change in which free bromine has been liljerated ; the 

 colour and odour are due to the presence of this element in 

 the free state. There is yet another element present in sea- 

 water very similar to bromine, but this exists in the merest 

 traces — its name is iodine. Its extraction is, however, per- 

 formed for us by certain sea-weeds. These absorb the com- 

 pound of sodium and iodine, and store it in their tissues ; 

 the sea-weed is dried in the sun and afterwards burnt, the 

 sodium iodide can then be dissolved out from the ash by 

 water. The three elements — chlorine, bromine, and iodine 

 — to whose presence in sea-water we have referred, form a 

 most interesting group, to which, from their forming bodies 

 very much like salt, the name of "halogens" has been 

 given. We propose in our next paper to describe some 

 experiments illustrative of their properties, and through 

 these to explain the modes of their extraction from sea- 

 water. 



THE FISHERIES EXHIBITION. 



NATURAL HISTORY DEPARTMENTS. 

 By John Ernest Ady. 



IN his interesting preface to the Natural History Section 

 of the Exhibition, Dr. Francis Day makes the follow- 

 ing observations.* 



" The naturalist and fish-culturist ought to approach the 

 question from an entirely different point of \-iew ; he 

 should ascertain the^life-history of all forms of fishes, — not 

 merely such as ser\"e for human food, but also of those 

 which form the sustenance of the more predaceous kinds. 

 He should endeavour to collect reliable information respect- 

 ing the plants and animals which afford them sustenance 

 and shelter, as well as ascertain what are their enemies or 

 their friends, and what conditions favour the presence or 

 absence of either class. Irrespective of the foregoing, he 

 should consider the relationship of temperature, currents, 

 soils, and the various conditions of the water in which 

 they reside ; to their migrations, growth, health, and 



* " Official Catalogue, Great Inteniationul Fisheries Exhibition, 

 London, 1S83." 1st Edition, pp. IGO, IGl. 



