Periodicity of Freshwater Algce in Nature 161 
that the rather wide fluctuations (between 12 and 20 degrees) in the 
total amount of dissolved matter of the water (Fig. 3) during these 
dates is very probably due for the most part to variations in the 
quantity of dissolved organic matter, and not of the inorganic salts. 
A single experiment was made to determine roughly the amount 
of organic matter in the water by the method of ignition 1 . On 
September 18th, 1920, a known weight of the deposit, obtained by 
evaporation over a steam-bath in the ordinary way, was heated to 
redness on a piece of platinum foil, and the loss in weight ascertained 
by weighing. The original weight of the deposit was 0-026 gram, the 
residue after ignition weighed o-oio gram, the loss in weight (re¬ 
presenting chiefly organic matter) thus being 0-016 gram; or, in 
percentages, roughly 61-5 per cent, organic matter and 38-5 per cent, 
inorganic salts. On the same date the total amount of dissolved 
matter was 13 degrees (= 0-013 P er cent.), and from the above 
experiment we see that roughly 8 degrees of this represents dissolved 
organic substances, and 5 degrees inorganic salts. Now since on this 
date the total hardness of the water (by the acidimetric method) was 
4-5 degrees there remains only 0-5 degree to be reckoned as inorganic 
salts other than those responsible for the “hardness” of the water. 
Thus it is probably correct to say that only 0-2 to 0-4 of the 
deposit, as obtained each month by evaporation over the water-bath, 
was composed of inorganic salts, the rest being organic matter; and 
that while the inorganic part of the deposit varied from month to 
month apparently between comparatively narrow limits, the amount 
of organic matter fluctuated between much wider limits; so that 
the marked variation observed in the total dissolved matter of the 
water during the period of observation is mainly to be accounted 
for by this rise and fall in the amount of soluble organic matter. 
V. THE ALGAL FLORA OF HAWKESLEY HALL POND 
The pond is very rich in species of Algae, and the list has been 
divided into three groups: (1) those Algae which at certain times 
have formed a really conspicuous feature of the algal vegetation of 
the pond; (2) those which have developed at times only to a moderate 
extent, or have been only locally common at times; and (3) those 
which have been rare or of isolated occurrence. 
Group i includes: Cryptomonas ovata, Trachelomonas volvocina, 
Chlamydomonas Reinhardi, Eudorina elegans, Pandorina Morum, 
1 This method, of course, is not very accurate for the determination of 
organic matter in water, but it will serve to give a rough idea. 
