OctoBeErR 25, 1918] 
out by Gillespie? and by Sharp and the 
author.*® Recently Plummer’ has confirmed 
these views and presented much additional 
data. It is often assumed that most agri- 
cultural plants require a slightly alkaline re- 
action in the soil. Previous work has shown 
that a reaction of Px 5.0 is in no way in- 
hibitive to the growth of barley seedlings. 
This point has been investigated further, with 
the use of several varieties of beans in sand 
cultures. The solutions were changed with 
sufficient frequency to maintain constantly an 
acid reaction. No evidence of injury was ap- 
parent. Truog?® upholds the view that the 
acidity per se is not ordinarily the limiting 
factor in acid soils. - 
With reference to the latter point it may be 
of interest to present some observations made 
on an area of California peat soils. All were 
found to be decidedly acid, as follows: 
TABLE IV 
Py, of Soil Suspensions 
Description of Soil Pau 
1, Surface, never cropped ......... 5.4 
la. Subsoil from above ............. 6.0 
2. Surface, cropped 15 years ...... 4.9 
2a. Subsoil from above ............ 4.9 
3. Surface, 2 years in potatoes ..... 5.1 
3a. Subsoil from above ............ 4.6 
4. Surface, barley field ............ 4.5 
In sections where other inhibiting factors 
were absent first class crops of barley, oats, 
beans, potatoes, onions, corn, asparagus, etc., 
were produced. Incidentally it may be men- 
tioned that from 100 to 3,000 p.p.m. of NO, 
(basis of dry soil) were found. It is evident 
that in these soils the acid reaction did not 
interfere with the growth of crops nor the 
formation of nitrates. 
D. R. Hoacranp 
DIvIsIoN OF AGRICULTURAL CHEMISTRY, 
CALIFORNIA AGRIC. EXPERIMENT STATION 
8 Gillespie, L. J., Jour. Wash. Acad. Sci., Vol. 6, 
No. 1, pp. 7-16. 
® Hoagland, D. R., and L. T. Sharp, Jour. Agr. 
Res., Vol. XII., No. 3, pp. 139-148, 1918. 
10 Truog, E., Soil Science, Vol. 5, No. 3, pp. 169- 
196, 1918. 
SCIENCE 
425 
AN ELECTRO-THERMO-REGULATOR FOR 
WATER BATHS 
THE majority of electro-thermostats so far 
devised and in use in paraffin ovens, incuba- 
tors and control chambers have been con- 
structed to operate directly in the chamber 
itself. Such installation is not wholly de- 
sirable, inasmuch as a regulator so placed is 
subjected to sudden and extreme variations of 
temperature each time the door or compart- 
ment is opened. This not only interferes 
with the stability and accurate operation of 
the instrument itself, but causes an unnec- 
essary number of “makes” and “ breaks” on 
the part of the mechanism, with its attendant 
corrosion of the contact points, and is also 
conducive to fluctuations in the temperature 
eaused by the unnecessary heating and slow 
cooling of the heating element. The thermo- 
regulator herewith described is designed to be 
inserted in the tubulature of the incubator, 
where it is immersed in the water of the water 
jacket. It is intended to be used in connec- 
tion with a secondary switch in circuit with an 
electric heating element. So employed, and 
with a moderate current passing through the 
primary circuit, the thermostat will give con- 
tinuous control within a fraction of a degree 
of the specified temperature. 
The thin brass casing C’ is 29.5 cm. long by 
2.5 em. outside diameter. It is strengthened 
at its upper end by a collar D, which extends 
on one side to form a rigid arm A. Binding 
posts B and B’ are for wires leading to the 
secondary switch, which is placed in some out- 
of-the-way situation. They are thoroughly in- 
sulated from the arm A by red fiber composi- 
tion. Binding post B’ is bolted securely to a 
curved saddle-piece S, in which is suspended 
a bent lever Z. Platinum points P and P’ of 
generous size are soldered to binding post B 
and lever L respectively, while a no. 3 cover 
glass 7’ is cemented with hard shellac to the 
vertical arm of the lever, insulating it from 
the rest of the mechanism. 
The thermostat operates on the principle of 
the unequal expansion of two different metals 
composing a couple. Zine and iron make a 
sensitive combination; but lead and iron, alu- 
