NO MORE SCURVY      A new vitamin called "Vitamin C," which is claimed to be an absolute cure and preventive of scurvy, has just been discovered by a French biochemist. He obtained the vitamin in crystalline form from cabbages. It is not a startling discovery since efficacious remedies for scurvy have long been known. The disease can be prevented or cured by an abundant diet of fresh vegetables or fruit or the juices of the latter such as lime juice. But the chief value of the new remedy is that it is very potent even when taken in minute doses.      The efficacy of "Vitamin C" is vouched for by the Washington Science Service, which takes great care to satisfy itself as to the claims of new, curative agents before it sets the seal of its approval upon them.      Authorities have been at variance as to the causes of scurvy, but it may be due to the absence of what is now known to be its cure - the vitamin which has just been extracted from cabbages. Since cabbages are cheap and plentiful and the method of extraction of the vitamin comparatively easy, it is evident that a new sense of security will be afforded to future explorers and castaways by the knowledge that they have a prophylactic against a disease which, in the not distant past, took terrible toll of the world's venturesome ones.