Despite the title, this is a short story, and actually on the shorter side of short. It was originally published in 1895 along with two other interwoven stories in a work called "The Three Imposters." I have not read the collection but this piece stands on its own as a short tale. The story is simple. An Englishman from the upper class is studying for the law and begins suffering from what might be called "nervous exhaustion." He is prescribed a white powder by his doctor and at first he seems full of renewed energy and vigor. However, the powder begins to take an awful toll and the man becomes more and more reclusive until...well, you'll need to read to find out. The story is told by his concerned brother, and in the course of the tale we find that the medicine prescribed by the doctor is not what the pharmacist supplied. As the basis of the drug, the pharmacist used a container of powder that had been on his shelves for many years and had been chemically altered by that long exposure into another substance called Vinum Sabbati--a witch's brew.
The story is pretty simple but effective. Since it's told by the brother, we don't "see" or experience the man's transformations except second hand. This was a common storytelling technique in those days and is still used today, although not as commonly. However, the writing is very fine and we get a good sense of mounting dread from the story. One can see how this tale was likely a strong influence on H. P. Lovecraft and his nameless horrors.
I suspect that Machen's influence here came at least partially from the writings of Sigmund Freud on Cocaine, which mostly appeared between the years 1884 and 1887. The drug was well known by the time Machen wrote this story, and quite a few doctors and researchers had extolled its virtues, although it's less desirable effects were also becoming known.