For the nineteenth and the early twentieth century, the Philosophical Transactions was issued annually, and at times bi-annually, but there was never a fixed publishing date. The full volume was available once all the papers received, read, and sent to the printers during the Society’s session (November-November) were available. From 1887, however, separate papers were available for Fellows as soon as they were printed. In the 1930s the Transactions begins to be issued in several parts per year (but not quite monthly). It is not until 1990 [true?] that the Transactions becomes a monthly publication.
The publishing of the Royal Society’s Philosophical Transactions underwent considerable transformation in 1886 when a Publications Committee was appointed ‘to consider and report to the Council upon the mode and regulations of publication at present adopted by the Society, and what changes, if any, may be advantageously introduced’. The Society was facing increasing specialization in science and continued growth in submissions. The Committee consisted of the Officers of the Society (President, Secretaries, and Treasurer), as well as several past presidents. Advice was also garnered from the Presidents of the Linnean, Geological, Zoological, and Chemical Societies.
Three months later, the Committee returned with recommendations. After months of discussion by the Council, they adopted several of its changes. The most significant was that the Transactions was to be split into two separate series, one physical (A) and one biological (B). The motivation was not stated, but it seemed that the Society was struggling to keep on top of the growth in submissions, and the increasing specialization of science. Splitting the journal made sense if the Society hoped to attract authors who were eager to publish in a more specialized journal. Yet, splitting in two still meant the Transactions maintained its relatively unique attraction as a general research journal. It also meant that Fellows could now receive one series of the journal depending on their research and interests. In fact, papers were from this date published separately (and only later in volumes), meaning that by the late nineteenth century the separate paper had surpassed the published volume to become the main mode of scientific communication.
Source: Council Minute Papers/6: 2 February 1887, p123-124, Royal Society Archives, London.