June 15, 1916] 



NATURE 



the outlet, the temperature in the burning zone being 

 approximately 1370° to 1650° C. This temperature is 

 gradually reduced until at the inlet end it is 3 15° 10455° ^• 

 The slurry is first dried by the hot issuing gases, then 

 water of combination is driven off and organic matter 

 carbonised; the dehydrated clay and lime gradually 

 approaches the clinkering zone, where at 1540° to 

 1650° C. the combination of the Hme, silica, and 

 alumina takes place. The clinker thus formed con- 

 tinues to travel down the kiln and drops into the cooler 

 as a white hot mass of small nodules. As these pass 

 down the cooler the incoming air abstracts heat from 

 the clinker, and thus receives a large part of the heat 

 necessary for combustion. Each ton of clinker burnt 

 requires about 5 cwt. of fuel. 



Formerly the shaft or chamber kiln was used, but 

 the rotary type of kiln is now almost universally 

 adopted in modern plants of any size, owing to better 

 burning of the clinker, greater output, and economy. 



The clinker is finally ground to an impalpable 



come up draw some of their food material from the 

 soil, and they build up their leaf and stem tissues 

 partly out of this and partly out of the carbon dioxide 

 in the air. The process requires that energj- 

 should be put into it; in this case the energy 

 comes from sunshine, and as neither energy nor 

 matter is ever destroyed in natural processes they 

 are added to the mineral matter of the soil after these 

 plants die, and their leaves, stems, etc., become 

 mingled with it. 



Direct exp)eriment shows that this addition of plant 

 residues is beneficial to plant growth, and it is now 

 known that the difference between the surface and 

 the subsoil lies largely in the presence of residues 

 left by generations of plants that have lived and died 

 there. The problem is to find why the plant residues 

 are so beneficial. 



These plant residues contain carbon and oxygen 

 in large proportions, hydrogen and nitrogen in 

 smaller proportions, and lesser quantities of phos- 



FiG. 2. — View taken from the kiln firing platform, showing the two 200-feet kilns, slurry feed -apparatus, dust chambers, and 



chimneys at Aberthaw. 



powder, the grinding arrangements being similar to 

 those for the raw materials. The Aberthaw works 

 produce 2400 tons of cement per <veek. J. A. A. 



THE SOIL AND THE PLANT. 

 Nature's Cycle and Man's Control.* 



T T is a familiar observation that the upper layer of 

 A the soil alone is well adapted for plant growth, 

 the underlying material or subsoil being wholly un- 

 suited for the purpose. But this distinction did not 

 always exist. When the soil was first laid down it 

 was all like the subsoil ; something, however, has 

 happened to bring about the change. Observations 

 on land slips and cliff falls, and direct experiments, all 

 show that whenever subsoil is left exposed to the air 

 it begins to cover itself with vegetation, the seeds of 

 which are blown or carried on. The first plants that 



1 Summary of two lecture: delivered before the Royal Institution on 

 February 29 and March 7 by Dr. E. J. Russell. 



NO. 2433, VOL. 97] 



phorus, calcium, magnesium, potassium, etc. The 

 chief reaction in the soil is an oxidation ; oxygen is 

 absorbed and carbon dioxide given out in approxi- 

 mate equal volume. The carbohydrates of the plant 

 disappear very rapidly; some of the cellulose takes 

 longer and gives rise to the black humus familiar to 

 all gardeners. The nitrogen appears as nitrate. This 

 last is not quite w^hat one would expect. In the 

 decomposition of protein as studied in the laboratory 

 the result is always a mixture of amino-acids. Under 

 the action of putrefactive bacteria the decomposition 

 is carried a stage further, yielding ammonia and other 

 bases, but nitrates are not found by the processes of 

 the chemist. At first sight, therefore, the laboratory 

 decomposition appears quite distinct from that in the 

 soil, but close study shows that this is not so. Repre- 

 sentatives of the groups isolated in the laborator>' can 

 be found in the soil, and, what is still more to the 

 point, if a trace of chk)roform or toluene is added to 

 the soil no nitrate is formed, but ammonia accumu- 

 lates instead. When a trace of untreated soil is added 

 the process starts again, and nitrate is found as 



