In a previous communication we have summarised the results obtained by us in a study of the nature and extent of atmospheric pollution in different parts of the city of Leeds and in further studies of some of the effects of smoke upon plant growth.

In two previous communications a summary has been given of the results obtained in determinations of the relative degree of atmospheric pollution, firstly in various parts of the city of Leeds, and secondly in the surrounding semi-urban and rural areas, to a distance in some directions of seven miles from the centre of the city. The results obtained in the latter series indicated clearly the presence of extensive atmospheric pollution, showing the characteristics of coal smoke, in all parts of the area investigated. The degree of pollution was found to fall rapidly on passing in a northerly direction from the centre of the city into an area free from smoke-producing industries, but less rapidly on passing into similar areas to the north-east and east of the city, owing to the greater dispersion of the city smoke in these directions by the prevailing winds. On the opposite side of the city, from north-west round by south to south-east, smoke pollution was found to be very high in all quarters.

Evidence is brought forward to show that free pentoses exist in the alcoholic extracts of foliage leaves of different plants. Their amount can be estimated with a fair degree of accuracy by the ordinary distillation process or by the reducing power of the purified liquor after other sugars have been fermented away.

When, however, small amounts of pentose have to be estimated accurately in presence of large quantities of other sugars, it is advisable, as suggested by Kluyver, to ferment away these sugars before applying Kröber's process.

It is shown that contrary to the statements of Kluyver and others, maltose undergoes slight hydrolysis (to the extent of about 2% when 1% solutions of maltose are used) when heated with hydrochloric acid under Herzfeld conditions. It is preferable therefore to adopt 10% citric acid, under the conditions formerly laid down, in estimating cane sugar in plant extracts when maltose is likely to be present.

1. The characteristics of true partial sterilisation have been found to be common to a large number of antiseptics and are

(a) An initial decrease in the numbers of bacteria followed by a large sustained rise.

(b) The killing of protozoa and nitrifying organisms. In no case have we observed all the usual partial sterilisation phenomena without the death of the larger protozoa which occur abundantly in cultures made from our soils.

An interesting factor governing the nitrate content of a soil has been brought out in the course of the preceding work on the treatment of soil with volatile compounds.

The evaporation of water from the soil fractions “sand” and “silt,” from china clay, and ignited soil is a relatively simple phenomenon which can readily be explained by the known laws of evaporation and diffusion. The evaporation from soil is more complex, something being present which operates in making the relation between the soil and the soil water of a different and closer nature than in the case of sand, etc. The effect is not due to the soluble humus, for the removal of this material from the soil does not appreciably affect the phenomena of evaporation. Any possible effect of the insoluble organic matter is largely eliminated by the consideration that ignited sand and silt behave like the unignited material.

In a previous paper I have stated: “The monosulphide sulphur, therefore, appears to be essentially calcium hydroxyhydrosulphide CaHSOH with very minute quantities of calcium hydrosulphide CaH2S2. The solution of lime sulphur then appears to consist of calcium hydroxy-hydrosulphide, calcium thiosulphate, calcium sulphate with sulphur held in solution.”

I have now to amend this by adding “calcium disulphide” to the above.

The method of analysis is now as follows: Preparation of sample. 50 c.c. of the concentrated lime sulphur to be diluted to 500 c.c. with water.

In investigations on the effects of calcium carbonate and magnesium carbonate on nitrogen fixation by A. chroococcum in soils and in solutions, the following results have been obtained:

1. Calcium carbonate in mannite solution cultures acts only as a stimulant, never as a toxic agent at least up to and including a concentration of 2%.

2. Magnesium carbonate is sharply toxic in solution cultures in concentrations in excess of ·1% to ·2%.

3. Calcium carbonate in soil cultures is without effect up to 1·4% of the dry weight of the soil when it becomes slightly toxic in the Anaheim sandy soil.

The foregoing facts clearly show that the reaction of species to manures is not a simple function of the chemical influences of particular substances on the growth of individual plants. They prove, however, that the final adjustment reached by the various plants is profoundly affected as the result of competitive interaction between all the species which contribute to the herbage. Furthermore, it has been shown that if manurial plots are studied in the light of the type of grassland on which the experiments are carried out, that the influence of this competitive interaction can be fairly accurately gauged. The principles which regulate the resorting of species under the influences of a disturbed equilibrium are, broadly stated, as follows: if the changes in environment are not of a fundamental character, the tendency will be for plants near their maximum distribution to be decreased, for those near their minimum to be increased, and for those near their optimum to be increased or decreased within the limits of their highest and lowest cardinal figures.