March, 1913. 



KNOWLEDGE. 



101 



USE OF COPPER SULPHATE FOR PURIFYING 

 WATER. — Several years ago an account was given in these 

 columns of the good results obtained by using copper sulphate 

 to remove vegetable growths from large reservoirs in the 

 United States. Since then the process has been adopted in 

 many other places, and in the Journal for Gas Lighting, 

 1912, CXIX, 507, Mr. G. Embrey describes the results of his 

 experience with this method of purification at the Gloucester 

 reservoirs, the water of which had become coloured and had 

 acquired a fishy odour owing to the growth of Chara vulgaris. 

 Experiments showed that as much as one part of copper 

 sulphate per million could be used without rendering the water 

 poisonous to fish or to human beings, but in practice a third 

 of that quantity proved sufficient to destroy Chara and similar 

 low forms of plant life. 



This proportion of the salt was sprinkled over the surface of 

 the water in the form of a fine powder, which sank to the 

 bottom before dissolving. By this means the maximum action 

 of the copper sulphate was brought to bear at the place where 

 it was most needed. The water, after standing for a period 

 of three to seven days, had lost its colour and unpleasant 

 odour, and showed no copper remaining in solution. By 

 treating the reservoirs in February, when only the diatoms 

 were present, the subsequent development of Chara. and 

 confervoid growths was prevented. The trout in the water 

 were not affected by the treatment, but the destruction of the 

 vegetable matter caused starvation of the roach. The copper 

 added to the water was partially deposited in the form of red 

 cuprous oxide on the stones, and was converted into black 

 cupric oxide by exposure to the air. 



Higher forms of vegetation are not injured by such propor- 

 tions of copper ; for it was found that algae could be destroyed 

 in watercress beds without injury to the plants. 



THE MANUFACTURE OF CARBON BLACK.— In the 

 United States a sharp distinction is made between "lamp 

 black," the soot deposited when an oil or resin is burned in a 

 limited supply of air, and "carbon black," which is obtained 

 by applying a cooling surface to a burning flame. A descrip- 

 tion of the process used in a factory producing about ten 

 thousand pounds of carbon black a day is given by Mr. G. L. 

 Cabot in a paper read at the Eighth International Congress of 

 Applied Chemistry [Original Communications, 1912, XII, 13). 



The bulk of the carbon black in the American trade is 

 derived from natural gas, which is ignited as it issues from the 

 ground, and consumed in rotating burners which pass beneath 

 fixed cooling plates, upon which the soot is deposited. Most 

 of the attempts to produce the black from petroleum oil are 

 stated to have been commercially unsuccessful. 



Carbon black as scraped from the plates has a specific 

 gravity of about 1-7 after removal of the "air" with which 

 it is impregnated. It is much superior in tone to ordinary 

 lampblack, since it possesses a rich " bloom," which makes 

 it a highly-prized pigment for the preparation of the best kinds 

 of printing ink and stove polishes. Lampblack, on the other 

 hand, is mainly used for colouring leather, unvulcanised 

 rubber and oil cloth, and in the manufacture of paints. Its 

 chief source is tar oil, which when burned in furnaces of 

 special construction provided with cooling chambers, yields 

 from fifteen to thirty-five per cent, of its weight of lampblack. 



GEOLOGY. 



By G. VV. Tyrrell, A.R.C.Sc, F.G.S. 



GEOLOGY OF DARTMOOR. — An interesting Memoir 

 with this title has been issued by the Geological Survey, and 

 describes an area of two hundred and sixteen square miles in 

 Devonshire, including the greater part of Dartmoor. The 

 geology is, of course, mainly that of the great granite mass 

 forming Dartmoor, which is intrusive in rocks up to Carbon- 

 iferous in age. It probably forms a gigantic laccolite whose 

 upper surface slopes gently under the adjacent sediments. 

 Patches of contact-altered Carboniferous rocks have been 

 found a mile within the granite boundary, and these afford 

 data from which the general dip of the granite below the 

 sediments can be estimated. Near Petertavy and Marytavy 

 the angle is from twenty-three degrees to thirty-two degrees, 



and its smallness accounts for the great width of the meta- 

 morphic aureole in that district. 



The Dartmoor granite is a coarsely porphyritic rock with 

 large phenocrysts of white felspar which are arranged in 

 fluxion-streams near the boundary with the sediments. 

 Tourmaline is present in most parts. Topaz and cordierite 

 also occur, but much less frequently. The Dartmoor granite 

 is of a more normal type than the other western granites. It 

 is less acid and has suffered less from pneumatolysis. It may 

 be described as a biotite granite, or granitite with accessory 

 tourmaline, whereas the other granites from the West of 

 England usually contain abundant muscovite in addition. Its 

 felspars are of more basic composition, and oligoclase is 

 frequently present. 



The great mass of the Dartmoor granite and its slow cooling 

 has led to extensive contact-metamorphism in the adjacent 

 sediments. The average width of the contact-aureole is one 

 mile, and as a large variety of sediments of Carboniferous and 

 Devonian age are involved, a correspondingly large variety of 

 types of hornfels, spotted slates, and tourmalinized rocks occur, 

 and shew many different stages of recrystallization. On the 

 south and south-east the argillaceous sediments have been 

 converted to fine cordierite-hornfelses. Andalusite slates and 

 chiastolite slates also occur. A special belt of dark shales is 

 always the parent rock of the latter. The impure calcareous 

 sediments which are found within the aureole are especially 

 susceptible to contact-alteration, and give rise to a variable 

 series of calc-silicate hornfels, of which the most interesting 

 are the scapolite-bearing types. 



The spilite lavas and intrusive greenstones which occur in 

 the Devonian and Carboniferous rocks are intensely contact- 

 altered where they appear within the limits of the aureole. 

 The spilite-hornfels are fine-grained banded schists con- 

 taining abundant hornblende, with biotite, felspar, quartz, and 

 occasionally much epidote. In both the spilite and greenstone- 

 hornfels occur certain minerals, notably tourmaline and 

 axinite, which must be ascribed to pneumatolytic origin. 



IRON-ORES AND BAUXITES OF NORTH-EAST 

 IRELAND. — The iron-ores and bauxites, which form a 

 prominent stratified zone in the midst of the Tertiary basaltic 

 series of north-east Ireland have been described by Professor 

 Cole and his coadjutors on the Geological Survey of Ireland 

 in a recently issued Memoir. These rocks are coloured 

 bright red, and are conspicuous at the Giant's Causeway, 

 where they separate an upper series of massive columnar 

 lavas from a lower series of thinner flows. They were formed 

 by the weathering of the lavas during a quiescent period of 

 the Antrim volcanoes. The materials of the interbasaltic 

 zone are pisolitic iron-ores, laterites, and bauxite clays. The 

 typical downward succession in Antrim is (3) pisolitic iron- 

 ore; (2) "pavement," a material with a false stratification due 

 to coloured streaks of basaltic decomposition, and varying 

 from a siliceous iron-ore to a lithomarge ; (1) lithomarge, a 

 decomposed basalt still retaining the joint structures and 

 showing the spheroidal weathering of the original rock. The 

 pale bauxite clays in Antrim have been derived from rhyolites, 

 although the formation of bauxite from basic igneous rocks 

 has been proved in other localities. They overlie the pisolitic 

 iron-ore when they occur. The whole of the deposits may be 

 compared with the products of the process of laterization now 

 going on in many tropical regions, and resulting in the very 

 variable material known as laterite. The first chapter of the 

 Memoir, by Professor Cole, is a valuable summary of views 

 of laterite-formation. He comes to the conclusion that the 

 laterites and lithomarges of north-east Ireland must be 

 regarded as examples of soils and subsoils formed under more 

 or less tropical conditions in a region of seasonal rains. 



MICROSCOPY. 



By F.R.M.S. 



A RARE (?) ALGA. — Many species of the Oscillatorias are 

 among the most common examples of low plant life. Some of 

 them may be found in nearly every pond and pool of water, 

 on heaths and open spaces, while on wet muddy paths, at the 

 base of walls, and round trees they frequently show as a dark 



