Atkins — Factors affecting Rydrogen Ion Concentration of Soil. 397 



but, before following it there, a digression may be made eastwards to study 

 the banks and arable land bordering the cross-road. 



Here on the walls facing south the most inhospitable-looking crevices in 

 the mortar are occupied by Parietaria officinalis, and give a reaction of about 

 pH 8-8'2. The earth-covered tops of the wall give much the same reaction, 

 and are covered by Veronica arvensis, Uuphoriia Peplus, 2'araxcccum officinale, 

 Bellis perennis, Senecio vulgaris, with an odd clump of Malva sylvestris, and 

 further on Capsclla Bursa-/>astoris at pH 7'8 or thereabouts. Immediately 

 inside the wall in the grassy ditch were Convolvulus sepium, Lychnis vespertina, 

 and Sympliytuni officinale. The reaction both here and in the field was about 

 pH 7'6. The field contained potatoes in part, and was full of weeds, such as 

 Fumaria officinalis, Euphorhia, Helioscopia, E. Peplus, Polygonum aviculare, 

 Papaver Rhoeas, Veronica' arvensis, V. agrestis, Scabiosa arvensis, Brassica 

 campestris. 



On proceeding up the glen a by-road to the east is tanked hj P. fruticos^ls, 

 with C. sepium, Vrtica dioica, and some U. eurojxieus, and among the grass 

 V. arvensis, the reaction here being close to neutrality, pH 7 '2. 



Eound the stream in tlie glen the ground is in places marshy, with rushes. 

 Nasturtium officinale. Polygonum Persicaria, Taraxaciom officinale, the reaction 

 being pH 7'0. The water of the stream, however, was at pH 8'3, close to the 

 value for bicarbonate in equilibrium with the carbon dioxide of the air. Two 

 shallow wells beside it were at pH 6'4 and 6'6, due to"excess of carbonic acid, 

 as was shown by insolating the water with filamentous algae growing in it. 

 Photosynthesis then rapidly removed the acid, the alkalinity rising above 

 pH 8 in a few minutes. 



The hillside on the east was characterized by the abundance of Pteris 

 aquilina, with Ulex europaeus and Ruhus fruticosus. Here the soil was markedly 

 acid, pH 5 '6 or 5'4. - 



On the other side of the road was a ruined cottage and several old red 

 sandstone quarries. Eound the cottage the soil was alkaline, about pH 7'8, 

 and supported R. fruticosus, Oonvolvidus sepium, Urtica dioica, Symphytum 

 officinale, Anagallis arvensis. Polygonum' aviculccre, Viola tricolor. In the 

 quarries the scanty soil in the clefts and ledges was from pH 5'6-6'0. In 

 testing these acid soils without standard solutions use was made of methyl 

 red, di-ethyl red, brom cresol purple, and brom thymol blue. Memory of 

 the tints these indicators give with standards, aided by Clark's coloured 

 chart, enables a considerable degree of precision to be obtained. The clefts 

 and ledges were occupied by U. europaeus. Cotyledon Unibilicus (which also 

 grows in among mortar at pH 8), Silene maritima With., Sedum a7iglicum, with 

 one clump of Erica cinerea. The lower parts with more soil had Prunus 



