PORPHYRACE^. 



[ 529 ] 



POTTIACE^. 



in fours, the contents of jnu'ple or red colour. 

 Fructification consisting of — 1. scattered 

 sori of oval spores ; 2. tetraspores (crucial) 

 immersed in the frond; and 3. antheridia, 

 on the same or distinct plants. P. laciniata 

 and vulgaris are common on our coasts. 



BiBL. Harvey, Brit. Mar. Alg. p. 261. 

 pi. 25 A; Thuret, Memoir es de la Socieft de 

 Cherbourg, ii. 1854, Ann. des Sc. nat. 4 ser. 

 iii. p. 5 ; Derbes and Solier, Supplement aux 

 Comptes Rendus, i. 



PORPHYRACE.E.— AtribeofFlorideous 

 AlgEe (according to Thuret), of low organiza- 

 tion, forming Ulvoid membranous fronds or 

 strata of Confervoid filaments, of a purple or 

 red colour. They are placed among the 

 Ulvaceae by most authors, but diifer in the 

 absence of the zoospores and, according to 

 Thuret, the presence of tetraspores and an- 

 theridia. They are marine, Porphyra grow- 

 ing on rocks and stones, Bangia the same, 

 or parasitic upon Zoster a, Algae, &c. 



Synopsis of British Genera. 



I. Porphyra. Frond plane, membranous, 

 very thin, of a purple colour, with oval spores 

 in sori, and tetraspores (square) scattered all 

 over the frond. 



II. Bangia. Frond fihform, tubular, com- 

 posed of numerous radiating cells in trans- 

 verse rows, enclosed within a continuous 

 hvaline sheath. 



' PORPHYRIDIUM, Nag. = Palmella 

 cruenta ? 

 PORRIGO. See Favus. 



POTASH , AND ITS SALTS. 



Caustic Potash. — The strength of the solu- 

 tion may be that of the Liq. Potassse of the 

 Pharmacopoeia. But we prefer a stronger 

 solution made with 1 drachm of the potassa 

 fusa or stick-potash of the shops, and 1 liquid 

 oz. of water. The solution should be allowed 

 to settle, and the clear portion poured off 

 into one of the test-bottles (Ixtr. p. xxiii.). 



Some remarks are made upon tlie action 

 of potash in the Intr. p. xxxviii., and others 

 under the heads of the tissues, &c. On 

 treating organic substances with this re- 

 agent, the cystic -oxide-like ciystals of the 

 carbonate (PI. 6, fig. /*) will frequently be 

 formed. 



Chromates of Potash. — The bichromate is 

 used in the preparation of the chromate of 

 lead for injection. Its crystals polarize well. 

 The neutral chromate is also sometimes used 

 for preparing injections. 



Nitrate of potash, nitre, or saltpetre. — 

 This salt is dimorphous : it usually crystal- 



lizes in six-sided prisms with dihedral sum- 

 mits, or in other forms belonging to the 

 right rhombic prismatic system. But some- 

 times it assumes the form of obtuse rhom- 

 bohedra, resembling those of nitrate of soda, 

 and referable to the rhombohedric s^^stem. 



The crystals exhibit very beautifully the 

 phaenomena of Analytic Crystals. 



BiBL. That of Chemistry. 



POTTIA, Ehr.— A genus of Pottiaceous 

 Mosses, including some of the Gymnostoma 

 and WeissicE of Hedwig and others. Wilson 

 separates as Anacalyptce the species with a 

 peristome (fig. 611). 



Fig. 611. 



s^»Sm:i 





M 



Pottia caespitosa. 



Fragment of peristome. 



Magnified 50 diameters. 



POTTIACEiE.— A tribe of Pottioid 

 Mosses. 



Synopsis of Genera. 



I. Pottia. Calyptra dimidiate. Peri- 

 stome simple or wanting; if present, com- 

 posed of lanceolate articulate teeth, simple 

 or with a longitudinal line, nigulose and 

 somewhat fleshy. 



II. Trichostomum. Calyptra dimidiate. 

 Peristome simple, sixteen teeth split to the 

 base into two cilia, or irregularly and there- 

 fore into more than two, erect, stiff, and not 

 tmsted. 



III. Barbula. Cahptra dimidiate-hood- 

 shaped. Peristome simple, ciliiform; cilia 

 thirty-two, solitaiy or approximated in pairs 

 on a more or less exserted basilar membrane, 

 split into two cilioles behind, very long, 

 articulate-rugulose, twisted to the left, rarely 

 to the right, in one or several spires, hygro- 

 scopic. Cells of the opercidum and calyptra 

 twisted in the same way. 



IV. Ceratodon. Calyptra dimidiate. 

 Peristome simple ; teeth sixteen, connate at 

 the base into a cellular membrane, split into 

 two long, nodosely articulated, dark-coloured 

 arms, paler on each side, densely trabecu- 

 lated at the lower part. Capsule thick- 

 skinned, shining, nodding, with a somewhat 

 nodose collum ; annulate. 



V. Weissia. Cahi^tra dimidiate. Peri- 

 stome simple or wanting ; if present, com- 



2 M 



