100 



KNO^ATLEDGE ♦ 



[March 1, 1889. 



of the lava conduit, causing the teiTific boiler-like explosion. 

 In New Zealand, the water was probably that of Lake 

 Rotomahana, which was emptied during the eruption; in 

 Krakatoa it was doubtless from the sea. 



In general the difference between a comparatively gentle 

 eruption like those in Hawaii and these more explo.sive 

 outbursts is probably caused by : (1) The difference between 

 the viscosity of the lavas. (2) The height and strength of 

 the cone. (3) The greater or less abundance of the water 

 supply. 



The lavas of Hawaii are remarkably glassy and fluid ; 

 while those of Vesuvius are much more infusible and 

 viscid. With regard to the second point it is evident that 

 the strength of the mountain may be caused by its form or 

 its material. Fissures open with comparative ease on 

 Mount Loa, though its slope is much less steep than that 

 of many volcanoes ; its material is mainly basaltic, and to 

 this its weakness is due. When a mountain is of great 

 strength it is clear that much greater resistance is oflered to 

 the forces within, and when this is the case the eruption, 

 when it does occur, will be more violent. With regard to 

 the supply of water, it is probably far more scanty in 

 Mount Loa than was the case at Krakatoa or Tarawera. 

 In Vesuvius it is mainly the greater viscosity of the lava, 

 causing greater resistance to the explosive force, that pro- 

 duces the violent projectile discharges from the summit. 

 In Mount Loa the eruption frequently commences by 

 activity in the summit crater, but this ceases when the 

 pressure is relieved by the opening of fissures lower down. 



The variety in the nature of volcanic products may 

 evidently be caused by the different heights to which they 

 are ejected, as well as by different rock material. At 

 Kilauea, where the ejections seldom reach a greater height 

 than 50 or 60 feet, the lava falls in a still fused state, while 

 in volcanoes whose eruptions are more explosive it is cooled 

 into scoria, pumice, or cinders. If the explosive force is 

 very great, as at Krakatoa, the lava is blown into enormous 

 bubbles, whose fragments are carried to immense heights. 

 Where the water supply is very abundant mud is discharged, 

 as at Tarawera. 



It is remarkable that although Mount Loa and Kilauea 

 are so near together, they appear to be quite independent, 

 and there is no correspondence between their eruptions. 

 Professor Dana considers that they support his conclusion, 

 expressed many years ago, that volcanoes are no safety-valves, 

 for, " if, while Kilauea is open on the flanks of Mount 

 Loa, lavas still rise and are poured out at an elevation of 

 10,000 feet above it, KUauea is no safety-valve, even for the 

 area covered by the single mountain." 



NITRATES. 



By D. A. Louis, F.I.C, formerly Assistant Chemist in the 

 Ruthamsted Laboratory of Sir John Lawes. 



GOOD deal of public interest has recently 

 been aroused with respect to the large 

 deposits of nitrates found on the coast of 

 Chili, and to their use in agriculture. The 

 necessity for high farming in England and 

 on the Continent, and the consequently 

 large amount of produce taken off the land, 

 calls nowadaj's for a correspondingly high 

 return to the land of valuable constituents in the form of 

 fertilisers, of which nitrates are perhaps the most important. 

 The class of substances known as nitrates are, chemically 

 considered, compounds of nitric acid, which itself is a com- 

 pound consisting of the three elements— hydrogen (H), 



oxygen (O), and nitrogen (N), combined in such proportions 

 that every 100 pounds of nitric acid contains If pound of 

 hydrogen, 22i pounds of nitrogen, and 76^ pounds of 

 oxygen. It is represented in chemical symbols by the 

 formula HNO3. When this acid comes in contact with a 

 base such as a metallic oxide, the hydrogen and the metal 

 change places, with the result that an oxide of hydrogen 

 (water — HjO) and a compound of the metal with the 

 nitrogen and oxygen of the nitric acid are formed, the latter 

 compound being a nitrate. Such a change may be repre- 

 sented in the following manner : — 



Na^O + 2HNO3 = 2NaN03 + H^O 

 Sodium Nitric Sodium Hydrogen 



oxide acid nitrate oxide (water) 



Nitrates are also formed by the action of nitric acid on many 

 metals and on metallic carbonates. 



Many nitrates are substances with which everybody is 

 more or less familiar. Such, for instance, are : — Silver 

 nitrate ( AgNOj), the lunar caustic used by surgeons ; stron- 

 tium nitrate (Sr(N03)2), used in pyrotechnical displays 

 for the production of red flame ; potassium nitrate 

 (KNO3), known also as saltpetre and nitre, used in the 

 manufacture of gunpowder, of which it constitutes 75 per 

 cent. ; there is also sodium nitrate (NaNOj), or Chili salt- 

 petre (otherwise known as cubic nitre, or nitrate of soda), 

 the valuable constituent of the " nitrate " we hear so much 

 about just now in connection with the western coast of 

 South America, where there are enormous accumulations of 

 it in the neighbourhoods of Tarapaca in Chili and Atacama 

 in Peru. 



Nitrates are frequently found in nature in other parts of 

 the globe, more especially in India, but also in Ai'abia, 

 Persia, Spain, and Hungary, and in small quantities they 

 are present in almost all soils. They originate from 

 decaying organic matter containing nitrogen ; plants die, 

 leaving their roots to rot in the ground whilst their stalks 

 and leaves fall on the surface and suffer decay. More- 

 over, in all parts of the world, animal deposits of various 

 sorts accumulate and suffer decomposition ; whenever such 

 decay takes place in the presence of air and of some base or 

 bases in course of time the nitrogen of the vegetable or 

 animal matter is converted into nitric acid, which forms 

 nitrates with the base or bases ; in some soils these changes 

 take place with great rapidity. In various parts of Europe 

 similar changes are produced artificially. Heaps are con- 

 structed of waste and decomjjosing vegetable and animal 

 matter mixed with cinders, chalk, marl, kc, which are 

 watered with animal liquors, and after a year or two large 

 quantities of calcium nitrate are extracted from them. 



Scientific investigations have shown conclusively that these 

 important changes are the result of the vital activity of 

 minute organisms whose appearance has, up to the present 

 time, not been desciibed by investigators so as to admit of 

 their detection by the eye ; either they are too small or too 

 transparent to be seen with the microscope, or for some 

 other reason they are not discernible. They, however, must 

 exist in their myriads in the air, and are still more 

 abundant in the upper layers of the soil. These little 

 organisms have apparently their likes and dislikes, for 

 they thrive vigorously in wai-m places, where there is 

 sufficient moistm'e and plenty of bases, nutritious matter, 

 air, and material to work on ; on the other hand, they 

 perish in the cold, and in the presence of many poisonous 

 substances, such as corrosive sublimate, and they work 

 badly in the light. But where conditions are favourable, 

 or have been favourable to the existence of these little 

 microbes, we find plenty of nitrates. 



In those parts of India where the soil is light, and contains 



